If you are working at a Galaxy server that is running the most current release, there shouldn’t be any problems. Even most older releases are not a problem.
But you can test that out – try only deleting (not permanently deleting) “History A” then working with the data in “History B”.
Copies (clones) of data within a single account – in different histories or not – do not consume extra quota space, and only the active version of datasets are accessible by tools. This should work fine at any of the usegalaxy.* servers (all are current), and if you are running your own and run into problems, try updating to see if that resolves problems.
Once confirmed to be working, you can purge the history not needed, and the copy/clone of the datasets in the other history will represent the master copy of the data.
ps: Thanks @wormball for the advice about the older gotcha