There is a research property, where you must have the prerequesite labs to continue building a ship or using an ability. This doesn't apply to all research topics.
If you were to change the max ship slots to be the same, wouldn't they follow the same rules? If you had rank 5 fleets, but fell down to 2 labs... it might work out.
Naturally you'd have to lose a LOT in order to have your research fall back like that, but the restored income might help you rebound.
