What I would do if I were you is to contact SmugMug and tell the concerns you have about file conversions. However, since SmugMug stores RAW images regardless of kind, I believe that HEIF images aren't converted to any other formats unless that is what you want them to do.
Maybe this can be of help to you (?)
The High Efficiency Image Format (HEIF) is used by Apple's iPhone and is also coming to Google's Android P.
www.howtogeek.com
As for me, SmugMug is a great place to store any of your photos. I store "web ready" JPEG photos to post on this and other forums at SmugMug, but a lot of people store their RAW, TIFF, and JPEG images as well. Their customer service is quite good, so I am very happy since they promptly respond via email to any questions I may ask of them.