It's actually really easy to get back some, if not most of your units.

As HH pointed out you can get black tickets that let you choose what unit you want, and there are quite a few of them to get too. Charisma will over fill, allowing you to burn whatever you didn't use last level up. Most of the old unit drop events are on a daily rotation with a 100% first time drop rate. Star rush reboots are a weekly rotation I believe.

Only real downside is everything in in Japanese and has censorshit. Other than that it's a lot more polished.