Will this Upgrade Work?

I have a 1.2 install that is what I am using at the moment.

In another directory, I am working on 1.5 and tweaking the layout and adding plugins etc…

Will I be able later when I am happy to just dwld my 1.5 and upload it to my 1.2 directory thus overwriting all of my 1.2 files but having applied the 1.5 plugins and layout changes.

I realize that any plugin that needs entries in the SQL table won’t work but I am hoping that some of my current plugins such as amazon Media manager, etc will be fine. I am not adding plugins to the 1.5 driectory that require me to add additonal SQL tables.

Thanks!