Well, yes ... 1.9 -> now 3.5 ... there are quite a few versions and changes ... 15 major versions and more 'point releases' (those last .#'s) between where you are and the latest/greatest.
Might want to strategize ... do a 'moodle march' or NOT do a 'moodle march'.
Not trival.
Lots to look at cause it involves upgrading PHP/MySQL .... and am almost willing to bet, the version of your server as well ... like Ubuntu 14.04 to highest ... or CentOS 5 to CentOS 7 ...
For Moode check out:
https://docs.moodle.org/dev/Releases
Like I said, lots to read and study.
'spirit of sharing', Ken