Well, the way I would proceed would be first with a communication around the change so users expect it; also advise them to not use the feature with the rate type (ask them to just ignore that part) whilst under the current policy so they can already get used to the idea and the change itself.
Then have a think if there are any changes you wish to apply to time cards created under the current policy so you can action them prior to changing the policy (as you stated above you needed to add some work notes).
If there are changes/additions on time cards you expect to be done by the users, instruct them to do so and give them a deadline.
We were actually in a similar position and a few weeks after updating the time sheet policy we had to recall some of the time cards in order to allow submission under the newest policy. Now depends how many time card users you have, but it should be a temporary inconvenience and then an ad-hoc request in case in a few month's time someone wants to amend a time card under the previous time sheet policy.
What I also did as an admin, for time cards in Pending or Submitted I cleared out any values in the Rate type field right after changing the time sheet policy (especially on time card records created recently) to save myself a headache later on; only because any change in State would have triggered an error message so wanted to be proactive on it.
As for the Q with copying the time cards, I cannot recall exactly if Rate type is picked up, though I would expect to; it is best to test this in a non prod or PDI if you haven't done so. If you have and it's picked up, then most you can do is just try to drive good user behaviour - advise your users to avoid using the functionality for that initial time card post policy change and also mention what would happen if they don't do that and how they can deal with the error themselves.
Hope this makes sense 🙂 Good luck!!
