This is an ongoing topic of discussion internally though we don't have any news to share at the moment. None of the solutions we have discussed so far are simple and clean.
The last one we talked about was a way to bulk-convert all the maps from an old-series tab into the current series, where it converts them to equivalent maps of the same tier, but the concern we had internally was that people actually wanted to keep their maps as the old series for collectibility. It would mean that when a new expansion comes out, you take out whichever ones you want to specifically have for your "old maps" collection, and then bulk-convert the rest of them and the tab to the new series and keep playing. Does this actually solve the problem properly?