But if they were never intended to be featured/speaking characters and only background, it's highly likely they don't have the level of articulation/control as characters like, say, Tomar-Re and Kilowog had...which would have to be made over from scratch again. They really couldn't if you had to get 100+ of them into the film.
But again, if they go with another effects house, say ILM or WETA...all those places eventually have their own proprietary systems and even software...so you'd be looking at a lot of time and effort (i.e. money) just transferring whatever existing models/designs there are, and adapting or rebuilding them for the new EFX house's workflow. Basically, 'recycling' models et al isn't the answer to saving money to such an extent to motivate a sequel. Even if it saves them anything, it could be easily offset and surpassed by increasing the actual intended screentime by just 1.5X the previous film.
The whole 'they'll do it cheaper' isn't what they need to sell WB on for a sequel, if they want the best chance to actually make it good. Something has to convince them that even though it will ultimately cost more, the money will be put to better use and result in a much better extravaganza of a movie. If you do sell WB on doing it for cheaper...then they'll force you to do it cheaper. They'll cut you off instead of putting extra money into effects like they did the last time. They'll sit there and expect bigger and better effects even though they gave you less money to spend on them....and if they don't get it, they'll fire you. By promising or agreeing to 'cheaper', you put an ultimatum on the production. Just the kind of open and positive atmosphere that a sequel needs to be really good, no?
