I have found that it is only the Park Life Pathways that do not have the upgrade option. The original vanilla ones do and any subbed asset pathways that use the vanilla ones are also upgradable but not ones that used the Park pathways. I cannot upgrade a regular paved path to the Amusment Park path but I can upgrade it to a gravel path (as an example). This applies to any of the Park Life paths. Not sure if screenshots help but I do have them showing the button missing and only with the Park Life ones.

I tried this with only this mod in use and it is the only time this is an issue. I went through to see if there was any issues with DLC's but none that I could find.

Yes, I could rebuild each one I want to change but well, I will just say it is a big P.I.T.A with tons of them to do that. Yet, loading times to switch mods on and off is a P.I.T.A too. So danged if I do and danged if I don't LOL