Though it wouldn't be the first time they changed an actor for a role.
But the whole point of doing a Q story would be to feature John de Lancie.
"Audiences, please note that, in today's performance, the role of Q will be played by..."
Well, that certainly goes back to de Lancie's "Days of Our Lives" roots.
And it would be different from Generations, when they needed Shatner for the "cross-over" movie... how?
They secured Shatner's participation early, IIRC. It was his seventh ST movie for Paramount. They did write barely-more-than-cameo roles for Nimoy and Kelley on spec before securing their participation and they turned them down.
So their parts had to be (barely) rewitten for Doohan and Koenig.