Here's an idea:
For the legacy toons who are having their drop rates doubled, require 1/2 the credits to level up. The credits necessary for a star promotion can remain the same or be halved as well. (Alternatively, only the promotion costs can be halved.)
This would mean that instead of 8+ million credits, older toons would cost 7 million + a bit (promotion cost halved), 5 million+ (if having level up cost), or 4 million + (if halving both).
Since it only affects older toons, and only those toons affected by shard doubling (thus not Legendaries, etc.), it still fits the theme of bringing up a wide-ranging roster quickly so that people can meaningfully participate in both Hoth TBs, but it doesn't affect the end game economy at all.
Since we're doubling most shard progress for new players, reducing the costs by 12% - 50% shouldn't ever give you more wealth at any stage of roster development than original players had. It just means that new players pass through those stages of roster development more quickly.