can i change from a solo loop to a sport band?

Yes those are both supported and pretty easy to change too. Here the exact steps:

Change your Apple Watch band - Apple Support

YES, you can indeed.

38mm and 40mm watch faces use the same band

42mm and 44mm use the same band

What Size Watch Band Does My Watch Band Take

In the Watch app on your iPhone go to My Watch, then at the top of the page tap All Watches. You will see the entry for your watch, including the case size. This tells you which band size you need to order, for 38/40mm or 40/44 mm case.