I would think that would be governed by the venerable legal principle, " The proof of the pudding is in the eating." 
If he truly changed his posting habits, never mentioned any federal policies, never mentioned his administrion, never used the twitter account to give operational instructions to the Joint Chiefs of Staff, never mentioned “fake news” as a response to posts about his policy, that would be significant.
And if he instead just used his twitter account like any other semi-retired granddad, to post about his golf game, brag about his grandkids’ performances in the school play, and the occasional kitty pic and snapshot of the sunset at Mar-el-Lago - in short, if he changed so it was a purely, truly personal Twitter account, that would be significant.
And, if any time someone posted about his administration, whether favourably or dissing, he had a stock reply: “This is my purely personal Twitter account and is not meant to discuss political issues. If you wish to post something about my Administration, please do so on the official White House twitter account.” That too would be significant.
But if he just posts that this is his personal Twitter account, and then carries on in the same way as he has since he was inaugerated, I would bet that the courts would say that it’s a functional test, and he’s still using it to communicate his political views and declarations of policy. Meaningless disclaimer cannot negate the First amendment.