From what I re-call, there weren't any major changes to moodledata directory (if any at all).
Since it is highly recommended to work with a copy of the production site, one could begin the 'moodle march' with the upgrade from 1.8 to 1.9.19+ (highest) via git ... copy of the data directory ... copy of the 1.8 DB and the 1.9.19+ code acquired via git. Once the code directory is under git, it's simple to move upwards - one just changes the track to the next version up and pull the code.
Good idea to do a site backup on the copy at each successful upgrade so once the 1.8 is moved to 1.9, do a site backup (that becomes fall back position so you don't have to start completely over).
Research any add-ons one might have in the 1.8 ... or custom themes or any 'hacked' code.
The biggest change is the upgrade from the 1.9 to 2.2.x, as you already know.
You'll have to upgrade your php and your MySQL if the goal is 2.5.1+.
And, highly recommend tweaking php (setting longer times for scripts to run, etc.) as well as MySQL (innodb engine/character set utf8/collation utf8 general_ci) and other settings such as connections increase and max_packets_allowed - even if they are not specifically mentioned in Moodle docs.
'spirit of sharing', Ken
