There are two reasons I don't like my current MP5SD.
1st, as some have suggested, it's a pain in the ass to clean. I would like to shoot the hell out of it, and not have to clean a suppressor. Yes, it is very quiet, but many times I don't care about suppression when recreation shooting. I'd much rather have the choice to shoot suppressed or un-suppressed, and have a serviceable suppressor when cleaning.
2nd, it would be nice to update with modern optics, lights, etc. There are no rails on my current host.
When comparing my full auto MP5SD to my semiauto Sig MPX, I sure wish I could own the full auto version of the MPX. Just saying!
I have an HK91, which I bought thinking someday I might convert it to accept the sear. Frankly, if I was going to go rifle caliber, I'd want a belt fed which would be more expensive to shoot. My current shooting options are limited to sub guns, so it would be hard to shoot anything other than sub calibers anyway. So 9mm makes sense to me.
So assuming I bought a new SP5, who are the reputable guys to do a good conversion, and any idea what the costs would be?