Upgrade from 7.5.4 to 8.6

[DRYALLS]

I've just had a very bad experience trying unsuccessfuly to migrate and upgrade from 7.5.4 to 9.0 [using both backup-convert utilities, and never loading the data into 8.6]

I've given that up. So now I have an old server RHEL3 PLESK 7.5.4, new server CentOS 5.2, PLESK 8.6.
I want to migrate all clients and domains to the new server.

1. Can people here give me the confidence that a 7.5.4 backup, converted via backup-convert will then install on 8.6?
2. Any 'red flags' I need to be aware of? Things that don't work, and should be deleted on 7.5.4 then manually added back into 8.6?
3. I'm concerned. I *think* that both the 7.5.4 and 8.6 versions need to have the same license key. Right now they don't. Is that going to be a problem, and how do I fix it?

Thanks in advance for any replys/advice.