The best way to de-age Cullen is to do what Starwars is doing with James Earl Jones. Have him record words, and lines and use AI to make him sound young.I say this as someone who grew up on G1:
They need to move on. Cullen and Welker are iconic but Cullen is 82 and Welker is 78. Neither sounds like they did in their heyday and Cullen in particular has lost a lot of the range in his Optimus voice. They need to find a way to solidify some new blood in the roles so that they can have people take those character voices into the future and not simply sound like they're doing a bad Peter Cullen impression (like the guy that's been Optimus in non-Cullen roles over the last decade).
Garry Chalk was good as Optimus Primal and as Unicron Trilogy Optimus because he had the same sort of energy and spirit about him without just sounding like a carbon copy. That's what they need to shoot for.
But in spite of saying this, if they do another Beast Wars-related show/movie that features T-Rex Megatron and he's not voiced by David Kaye I will burn their studio to the ground.
All that said, on the topic of this movie: I hope that kids like it but I'm not sure it's for me. It has strong "generic kids CG movie #5,239" vibes going on with that same sort of off-the-wall silliness and quippy meme-factory stuff that you got out of Minions or the Super Mario Bros movie. Except both of those are light enough fare that they can work with that. Transformers isn't really in that same vein.