Hi Michel,
As I mentioned before, a common way of working for film composition is actually to construct a tempo map in advance of doing any writing. This means that any tempos and tempo changes are already in the empty 4/4 (or whatever) bar structure even before any music has been written. So you start off with something like this (imagine something like this but much longer - stretched out in terms of bar length):
As I said, perhaps with dummy notes entered for certain points you want to hit in the on-screen action, just to test the synchronization. But there is no music at this point, although the tempo and tempo changes are fixed. I have a few different tempi and there is a 75% rit in bar 2 lasting 4 beats and ending at the downbeat of bar 3.
Next you would actually compose part of the cue and many composers may work on some important spot first and then lead up to that, so lets say our important spot that we work on first begins in m. 4 (in an actual score this would be more like m. 40 or so but that would be too big of an image):
Now I’ve got the ending to my cue and everything synchronizes up, and I want to write the beginning that leads to that glorious musical moment. But now that I’m actually coming up with ideas, I decide I want the beginning to be 3/4. So I make this change from bar 1-3:
I did not move the rit, it moved automatically when I changed the first bar to 3/4. I just selected it to clarify the location, as the rit that previously fell on the downbeat of bar 2 and lasted four beats now falls on beat 2 of bar 2 and lasts four beats until bar 3 beat 3, because it stayed in the same spot in terms of the overall musical time in beats. This means that even though I have changed this time signature, everything I wrote before in bar 5 (previously 4) and later falls at the correct time in seconds, because the rit still happens exactly 4 beats from the beginning (as it originally did) and is 4 beats in length.
This is exactly the way things work in DAWs and is a big part of the reason why Dorico is so viable for film scoring compared to other notation programs, an area where DAWs have generally reigned supreme.
Obviously making any musical rits or accels after the fact will throw things off, and so when these are added later for musical reasons, it involves some time and trial and error to get later events syncing up again the way they should in the last bit. Also you may decide that you want the rit to happen a bit differently so it begins on a downbeat in the final music for easy readability, assuming that moving the rit slightly won’t affect any hitpoints, and then you’ll similarly have to do some extra futzing with it, but this is the same as in a DAW.
The important thing is in both DAWs and Dorico, composers can feel confident that they can change time signatures, and thereby change where the bar lines fall, without impacting the sync of anything.