I have a phpBB 3.0.4 forum with both prosilver and subsilver2 based styles, with english and hungarian language and with a few mods installed.
Finally I think it's time to apply your SEO mods as they would be quite useful.

My question is: How should I perform the phpBB update, the style update and applying the premod as well, while I still want the forum to be working all time, so the update should mean ~1 hour downtime at max.

I'm planning on saving the whole database, manually removing every mod's database changes (to get a clean 3.0.4 db), uploading it back to the server (with a different prefix), installing a phpBB 3.0.4 forum, editing the config (so it uses the database uploaded before), updating it to phpBB 3.0.6, and uploading the phpBB 3.0.6 version of the styles I use.

By the time I should have a vanilla phpbb 3.0.6 forum with no mods.

Then I want to install the SEO mods, preferably the premod. I will still need to revise the code changes in the templates and in languages and than configure the mods (i do not plan to translate te mod in hungarian, just the parts viewable by plain users, but not the RSS/sitemaps)

Then I want to install other mods I want to use and see if they are conflicting with the SEO mods and repairing them if they are not working properly.

And when it's finished I need to move the new forum to overwrite the old one. and there I will need to change the path of phpbb in the config again (and possibly in some SEO mods(?))

AM I DOIN RITE?!

Sounds good.

[HOW TO] Install the phpBB SEO premod on phpBB should help too