Are external HDDs compatible with High Sierra without reformatting?

Hello community:

I have a late 2011-model MacBook Pro that will not be upgradeable to the upcoming Mojave. Right now, it's running Sierra, so I'll need to upgrade to High Sierra very soon.

Its internal/boot drive is a 750GB HDD (not a SSD). Because I do graphic design, illustration, video, and music, a huge chunk of resources and projects are on several multi-partitioned USB2 external hard drives that are also HDDs (not SSDs). I'm quite sure they are all formatted as HFS+, as I've only formatted them using my MBP.

My big question is: will I still be able to use my external HDDs as-is (without reformatting to APFS) if I upgrade my MBP to High Sierra? From what I understand, High Sierra won't reformat my internal HDD because it isn't a SSD or Fusion drive. Is this correct? Has anyone with a similar configuration encountered any problems upgrading to/using High Sierra?

(Before you ask, a new Mac is out of the question for financial reasons. 😟 )

Thanks everyone!