I’ll go along with WhyNot’s answer. I send out price increases to customers, and state it simply: These are our new prices effective this date. Please use them from the effective date on…or something to that effect.
If I want a coworker to do something for me, I’ll ask, “Would you…?” and not “Could you…” for the same stated reasons.
So it depends on context, and the relationship I have with the person.
Thanks so much for your business! We are asking that in the future, all of our clients utilize our webform for blahblahpurpose. If you wouldn’t mind, that would expedite blahblahprocess and let us help you even more efficiently/quickly/more cheaply/whatever imagined benefit.
I work in a call center and I would never use any of the three phrases you mentioned. They sound too commanding. Unless it’s a mandatory change you’re making. If it IS a mandatory change, then just say, “In the future you will need to blahblahwebform as our system is being transitioned into the new decade blahblahtechnology”
I feel the same way about the word please. It makes the statement a request, and thus allows you to say no. Just be nice in the statement, and you should be fine.
I prefer something like “From now on, you will need to blah blah blah.” If you want to soften that, just explain why:"Due to abuse, we are disabling our email-based support system. From now on, if you need support, you will have to use a the web form at website.com/contact.php.
“We regret any inconvenience.”
There seems to be this illusion that you can’t be both direct and nice at the same time. That’s only true in a few emotional situations: it’s not a general rule.
Sorry, rachelellogram. As a client, if I received this, I’d stab someone in the eye with a pen.
I think I could refrain from violence, but I would be thinking snarky thoughts along the lines of, “…and what if I *do *mind?!” :dubious:
When it come’s to clientele, I’ve found that clearly stating why and what you’re trying to do and why it’ll be mutually beneficial, that you’ll be fine so long as it sounds honest and professional.
“Please do X” is a straight-forward instruction. “Would you please do X” is more polite, but does not adequately carry the connotation of ‘this is the way to accomplish what you’re trying to do.’
I’m thinking of my mother-in-law, who is incapable of carrying out instructions without changing them. ‘Would you please not do the laundry while I’m out’ means that my hand laundry will be shrunk in the dryer when I get home. ‘Don’t do the laundry.’ might just save my shirts.
‘Could you do X’ is right out.
and brief. I really don’t care about your business reasons for the change. If Newprocedure is my only option, then, if you don’t mind, don’t be wishy washy and lengthy. 
Just the facts, ma’am.