The ID in the Edittime.js of the V1 and V2 plugins are different, which is why they are not backwards compatible.
Altering the ID might enable you to open the older version. Try changing:
"hmmg_layoutTransition_v2" to "hmmg_layoutTransition"
Obviously, MAKE A BACKUP FIRST ! This applies to both the project and the plugin.
I still have the V1 of the plugin if anyone really needs it. Just let me know.