upgrading ec3 to ec4

Hello,
I've just finished reading Selling online with Drupal Ecommerce (http://drupal.org/node/257057) and seem to have quite a few problems with ec3 - that weren't addressed in the book - namely taxes and some random shipping errors.

I've only upgraded a few modules in the past - none as complex as ec3 -> 4. Typically, I disable the module I want to upgrade, then upload the module to the site, then re-enable. However, it seems like a lot more has changed with ec4. Should i disable ALL ec3 modules, then upload ec4 as usual, or is it more complicated than that?

If anyone has done this before and can provide some general guidelines, I'd appreciate it.

Thanks,
kwg