what sort of changes are you referring to? TUPE is indefinite if changes are related to the transfer
as you and others have said a lot if vague but it's something you do not what to fall foul of
as I said working hours etc
to be honest you can do most stuff because nothing is directly related to TUPE

Such as harmonisation of pensions, working hours, bonuses. Its far easier to defend if it also involves changes to original employees as well.
Where you wouldn't want go really is pay, but Ive even seen that done, its just more tricky.
Most of your existing contractual terms and benefits, even pay can be changed with the correct notice period and consultation. As I said most of the time you don't go near the TUPE employees for a period, such as 90 days in order to avoid the "automatic" defence of it was related to TUPE. They may still try to pull this card at a later date, as long as you evidence there was a business need, such as to reduce costs, then your in quite a strong position.
By far the best solution is to get them to sign a new contract on your standard terms and conditions, its normally only an issue if there are very significant differences, but even then you can whittle them down, discretionary things such as pay rises and bonuses help to focus the mind when you don't get one due to signing a new contract and everyone who did sign gets them
