IN general terms, you can't change this as you can only make changes for Economic, Technical or Organisational reasons involving a change in the wider workforce (for example, if all your staff were on weekly pay and you were looking to move to monthly pay for everyone for a business reason). Even in that case, you would need their agreement

Changing TUPE'd people who are weekly onto monthly to come in line with the existing employees, seems to be a change that is very much connected with the Transfer (they wouldn't be changing from weekly to monthly if they hadn't transferred) and therefore isn't allowed.

You can try to get their agreement, but you need to be aware that even if they agree, they could later object and the change would be void as the change was connected to the transfer (i am pretty sure there is case law around this, but i can't seem to find it at the moment)

So you can try to get agreement, but need to be aware that there is a risk involved. Presumably the company would have known about the weekly pay issue when they agreed to take the contract/employees, so would have factored it into their costing etc during the commercial negotiations?