I just updated to UltraPak 2019, should I uninstall Band-in-a box Pro 2018 before installing complete UltraPak update on my iMac? Just duplicating and saving the folder containing my own songs produced with BIAB 2018.

You wouldn't need to do that. The way we set up our installers, if you install into your pre-existing directory it will consolidate anything and avoid making duplicates. Just make sure to install your 2019 version over your 2018 Pro directory and it should work smoothly.