HA Fillers can be dissolved with hyalase, even after a year of placement. Several sessions may be required to achieve the desired outcome, and sometimes controlling the amount dissolved might not be possible. As such you may need to undergo the dissolving, then a touch up with fillers a few weeks later.
Other fillers such as Non-HA fillers (like Radiesse, sculptra and Ellanse) are not really affected by hyalase and cannot really be dissolved.
Cheers!
Dr Shane Tan