If everything is working fine, "why fix it"?
There are advantages and disadvantages with v7. I am not aware of any security issues with v6.49.19 (but I have never used v6, except when I got my first RB760iGS, and then it was just for the duration of upgrading to v7).
As we go forward, there will be less and less info about v6. MikroTik has already thrown out useful info that was on the old wiki support site. See Old wiki.mikrotik.com vs help.mikrotik.com