from past experience, it’s potentially as big a step as going to SLES11(“SP0”) - and best practices for that were and are
What you will have to deal with is highly dependent on which subsystems you use. Major steps are kernel&libraries (so any programs compiled yourself or imported via not version-aware repositories need to be checked against those), but many other parts have significally progressed, too (Samba, openldap etc). If you i.e. run an old DC setup on samba, storing information in LDAP, and having other dependent domain members out there, you can prepare to do a bit of clean-up work after the update.
The following apply for every update, but the more for a major step:
Be prepared to have some extra free disk space during the update, though I have no numbers for you.
When having run the update, be sure to check for any RPM-archived config files (saved old configs or new configs that weren’t applied) and migrate the configuration manually.
Have a backup of the original system. Being able to access individual backed up files without having to restore the complete system is a definite plus.
Have at hand the phone numbers and/or email addresses of the relevant hot-lines, your boss, the local pizza service and those expecting you for supper: You might need extra help, and it may take longer than you anticipated.
Pretty generic stuff, I know. But it does match the question, even in this regard