I've performed upgrades from FreePBX all the way up to 2.8 as per the version of FreePBX with PlugPBX1.0 - no issues. Just upgrade with each incremental version and you should be fine.
IF you are paranoid or cautions, JUST cleanly shutdown your PlugPBX first, backup the SD cards contents (image it) and THEN perform the upgrade, if it all goes sour, you can just restore a prior backup and apologize to the wife, or other angry users
