SchulzStefan
Your question is somewhat vague.
If you mean is it OK to update 7.5.1 (with contribs installed) to 8b7 using a CD, then it should be OK, but keep in mind this is not currently supported, so something may break.
All contribs already installed before the update will still be there after the update.
Upgrades of clean installs (no contribs) of sme 7.5.1 to sme8b7 are supported and should work OK.
Developers are striving to have a smooth update from 7.x to 8.x but more testing is needed to ensure this works under all scenarios with various popular contribs installed.
You will also need to assess if any contribs you have installed are compatible (or not) with sme8.x.
Uninstall them first if they are not supported on sme8, to avoid problems.
Refer to the sme8 wiki for further info re contrib compatibility etc.
Personally I strongly suggest you test first ie upgrade a test server with all your desired contribs installed.
You can remove one drive of a RAID1 array in your live sme7.5.1 server and use that (in degraded mode) in your test server to perform the sme8b7 upgrade (with contribs).