Not quite.
The way it works right now: You enter the preshow as you progress in line (not wait for a 'next showing' to start). Though (IIRC) Footlight Parade is the 'first' trailer, it makes no difference as you experience them. If the line is short, you have the option of seeing less of it (though I always see them all). If the line is longer, the 'attraction' (IMHO) begins once you enter the theatre, so 25 minutes means shorter to the start then posted. The attraction starts with you entering the Chinese Theatre. As you progress into the 'theatre' proper, you see trailers. (Like today after all the commercials are played). Then for the main event, you LIVE the movies ('and what will be your fate' during the Indy scene). But it is centered around going to see movies the way they were intended to be seen - in a theatre.
How it may look after: You go into the Chinese Theatre, but the preshow is centered, not on the act of going to the movies: but turning on your television and watching Robert Osborne tell you what is coming up next on a Basic Cable channel. (They probably won't touch the 'theatre' decor of the theatre, but there's nothing 'theatric' about a Robert Osborne mini-lecture). And if it has a definite 'beginning, middle, and end' that they want everyone to see in full once it starts, it will make the wait even longer to see the attraction - all for a preshow that loses the dynamic of the trailers and no longer feels like it's part of the attraction. IMHO, whatever plusses the TCM 'brand' brings to the finale (and I think it will) will have a hard time making up for the lessening that happens to the start of the attraction - and it kind of messes with the theme of the attraction as a whole.
That said, it is good that 1) the attraction will still be here, and 2) they aren't going with the "it has to be nothing but films/franchises owned by $DIS" - TCM if applied the way they seem to may lessen the overall enjoyment and guest flow of the attraction, but it would be keeping it about classic Hollywood 'as a whole' - which the 'All $DIS owned' way decidedly would not, but would feel like a big commercial in a park that among other things, has a problem with layering it on thick in the 'pushing $DIS owned stuff' department.