I remember reading how the UK moved from its Shakespearian-era duodecimal ‘guinea’ system to the modern base 10 system, sometime in the 1960s? That was quite late considering the circumstances.
Nevertheless, given that people become attached to their customs, was there any resistance from the people to this change? Protests? Refusals?
What would happen if some American tourist brought back grandfather’s currency to the UK and tried to spend it? Would they even recognize a half-penny?
Yes, I was around at the time and the move was distinctly unpopular, especially since there was a settled conviction that shopkeepers were taking advantage of the change to subtly round prices up.
These days nobody under 40 or thereabouts has any memory of the old currency, so if you produced a halfpenny or half-crown to anybody under that age you’d be met with blank stares.
With anyone older though their face would light up as in recognition of an old and much-missed friend.
Edit: No organized protests, a few well-publicized refusals. It was generally realized that there wasn’t a lot the public could do about it, so we grimaced and we put up with it.
Some of us younger ones know all about it. Particularly if you read books written about pre-60s England.
IMHO that sums up the typical British response to a problem. We’re very inwardly very proud of our stoicism but few people would call it that.
Two nitpicks: firstly, decimalisation occurred in 1971, not the 1960s; secondly, it was not a guinea based system. The major currency units were the pound, the shilling, and the penny. The guinea was 21 shillings (£1 1s) and mainly used in auctions and races.
According to the BBC , it was mostly painless.
The Republic of Ireland decimalised on the same day, the Punt being linked to the Pound Sterling for some time at the rate of 1:1.
I miss the old Irish coins, despite only seeing them on holidays, the Giant Elk on the one Punt coin, the deer on the 20p piece, salmon IIRC on the 10p piece.
None of it is spendable any more, for a short time the old coins that matched a ‘new pence’ value were still usable which led to some confusion with conversions like sixpence = 2.5 new pence. Some of us older types would recocognise the coins. I’m just old enough to remember thrupenny bits and so on. Somewhere in the back of a draw I have a bunch of old shillings and sixpences but they stopped being real money years (OK decades :)) ago.
Even before decimilisation in 1971 our American tourist friend couldn’t have spent the half-penny (normally pronounced “hayp-nee”) as it stopped being legal currency in 1969.
I was working In Harrods on changeover day, and I had the dubious distinction of being a ‘Decimal Penny.’ My job was to wander around the store, trying to persuade all the rich upper class twits that Harrods wasn’t trying to screw them over by, for example, exchanging their old six pence with 2.5 new pence. Did they believe me? Did they hell! Have you ever been put down by a little old dowager who has never been able to do a sum in her life, and believes that Harrods - of all places! - should be above all this new fangled deception.
Shillings and two-shilling coins were actually used into the 1990s, since the original versions of the 5p and 10p coins that replaced them were identical in size and value. Of course, people didn’t generally call them shillings any more.
My great-gran was still giving me thruppeny bits for a treat up until 1974. I had to take them with a smile, and give it back to my mum, who would at some discreet point put it back in the ‘change jar’ it had come from.
They confused me slightly, more so when Dad explained the value of the old shilling in pre-decimal terms. Reminds me of the halfpenny piece that was around for a bit too, what a dinky little coin!
When I was very young, we still had farthings. Now, there *were *dinky!
The UK populace should have been reasonably well prepared for the changeover. I was 11 in 1971 and recall that every household was sent a ready reckoner showing how old money translated into new money. There was also a TV campaign, the most memorable Public Information Film being “Granny Gets The Point” with the late, great Doris Hare buying a quarter pound of tea.
And for some inexplicable reason, thickos suddenly started calling a penny “one pence”, which persists to this day.
But I certainly remember a great deal of confusion, particularly among the elderly in Ireland. My old granny, who died in 1975, never got used to the “dismal” (i.e. decimal) money at all. She could never fathom out how a shilling could now be “fippence” - where had the other 7 pence gone?? I used to try to explain that a penny was now nearly tuppence ha’penny but she refused to understand. The same thing happened - only 8 times worse - when Ireland switched to the euro.
By the way Pushkin, the Irish 20p wasn’t introduced until 1986 and bore the same horse on the reverse that used to be on the old half crown. The punt coin came in in 1990.
(for more on Irish coinage, see here.)
When I was very young we had groats…now they were big buggers. 
I hope they gave you some ointment for them.
One reason for this was that there was an eighteen month “changeover period” after decimalisation in Feb. 1971. All shops had the option of being “decimal shops” which only took the new currency or “£sd shops” which only took the old currency (but all prices in these shops were multiples of six old pence, to enable prices to be later converted to decimal). If you didn’t like the new currency you could continue to shop in the non-decimal shops for a while, until you got used to it.
Also, the 5p, 10p and 50p coins were introduced some years before decimalisation - with values of 1, 2 and 10 shillings respectively, so people were already used to them. (The 20p came later.) And the pound was unaffected - it was just subdivided differently. Its value was unchanged.
Ha’pennies (of the “new” variety) were still in use in Ireland when I was a child in the 1970s.
“LSD shops” has a distinctly different meaning on this side of the po(u)nd. 
How smoothly did decimalisation go in Australia & New Zealand? Both countries went even farther than the UK; switching from pounds, shillings, & pence to dollars & cents. They both made the switchover before the UK did. What about Canada? I’ve since Canadian one cent coins with George VI on them so it must’ve made the switch much, much earlier than the rest of the Commonwealth.
Noel Moore, the civil servant who masterminded the switch, only died a couple of months ago. The Guardian’s obituary explains how he spent a decade planning for “D-Day”.