Why can I not change the Project Dimensions for a project I open in Motion from within FCP?

Try this: with the copy open in Motion, select the Project in the layers list; in the Inspector, click Project, then click Snapshots.

Add a new snapshot by clicking the "+" button, and choose 9:16.

Save your generator.

This will not be totally perfect, but will give you starting version.

FCP will use the snapshot that corresponds to the aspect ratio of each project - so this new one will be used for vertical projects.

Making more tailored adjustments for this particular snapshot, so that it is looks better in your vertical projects, is possible, but is beyond the scope of this answer. Frankly, I detest vertical video, and I am not very well versed in working with these aspect ratio "snapshots"...

Is my issue too complex for a forum like this? That would be a perfectly fine response. I just wanted to ask, in case there is some... "Convert" button I might be missing.

In general, the answer is "no".

This particular *generator* is not really conducive to adaptation because the layers of "multiple" rectangular elements are consolidated into a single (png) image files (which you can find in the generator's template Media folder). Squeezing these elements to fit the vertical space, especially the slanted elements, would pretty much ruin the effect.

You can rotate the individual (clone) layers (the position keyframes should still work the same) but you will have a spacing problem (the distance gap of 1080 height over 1920 width).

You're best option would be to create individual *shapes* - possibly use a Replicator for the "far" and "mid" "groups" [to keep the "core" shape a single object] which gives you built in options for size variations among the replicated objects. The "drifts" are keyframed.... I would have preferred a rate slider.

At this point, the explanation does become complex as you would be building the effect from scratch and not the "composites" used in this original generator. And — there is *no, one, right way* to do this project.

I can tell you, the visual effect is nice, but the project was originally poorly built... looks like something a "programmer" would do, not a template designer. This can be built as a single shape and 3 emitters (no keyframes - built in animation — plus background material which can be reused). That would make it practically infinitely variable (with the right parameters published) for the end user.

So, since you would be basically starting over, how would you want to proceed? Copy it? Or make it better?

It doesn't take much to turn this:

into this:

(pardon my gifs)

You can get away with "squeezing" the background to fit. I masked a vertical section so you could see how the same horizontally formatted template can be modified to work in vertical space (just adding a checkbox selector), but a "Shapshot" may be a better option.

This is the old background from the original; 1 rectangle (emitter source); and 1 emitter. [Shapes position, size and speed can be randomized].

BTW - if this doesn't look quite right — I really wasn't paying attention to all the details for the various coloring options or the riggings. (I have my own way of doing things.)

Just looking at that same generator - opening a copy in Motion, in the presets menu you have custom. We can use

that to set the vertical layout.

Otherwise, just use the snapshot function that Luis pointed you at. This is actually exactly what I would do in this case, rather than change the project settings.