You may find this thread from a few weeks ago useful:
They were switching the other way (from accrual to cash) but the process and the potential pitfalls will be the same. Essentially you’ll have to make a big set of manual adjustments to your first return under the new accounting scheme in order to ensure that everything is accounted for once and nothing is double-counted, and keep a copy of all your calculations for that so you can prove what you’ve done in future.