1) do a complete, good, clean back-up (or two)....IF you don't already have one/two.

2) Unlock ALL returns

3) Proforma in blocks of 50 or so, with 2018 open to keep a check on the running total of clients.

That way, IF there's still a problem, you only have to look at a block of 50 for any that didn't proforma.

I did it that way on Wednesday, and had no issues.