As far as the infrastructure itself, we've seen the installation of the new pylons in the world showcase lagoon, the installation of utilities on islands in the lagoon, and the preparation for the new show equipment behind Africa.
RoE has the various barges that will need to be removed/dismantled, and even IF they had all the equipment built and installed for the next show, there's likely not enough space in the canal backstage to store both for a one-night transition like they do for normal fireworks shows.
Testing and etc. on the large scale of World Showcase Lagoon a la RoL is likely required too - so this provides some padding so testing can occur overnight while the temp show pacifies guests.
They could probably do a short turn-over (span of weeks to a few months) but it's not worth it to rush it like that. Especially when the temp show lasts less than a year, anyway.