Not sure if this has been suggested already, but here you go
Basically, select whether you want to TR to level 1 or level 20, then you get a list of the destinies you have any xp at all in & each has a drop down box with 3 options:
1 - Epic past life - This is the one you get the past life for. All XP in this destiny will be lost. You trade in the xp from a full life for a standard TR to get the past life feat, i don't think it's unreasonable to trade in the XP from the epic past life you want to get it's corresponding feat.
2 - Keep - Essentially locks the tree out of the TR process, XP progression in this tree would be retained as-is. No limit on the number of destinies you can choose to keep, so if wanted you can simply get your epic past life feat & lose no other destiny progress at all.
3 - Trade in for XP - Lose the tree progress, but gain an XP token (like the daily dice ones but BtoC) based on the number of completed levels within that destiny (probably about 75% of the XP required to hit that level, to prevent people levelling one they like then transferring it all to another less desirable one, you have to do some of the work!). The XP token can then be used at any time, so you could use it at heroic levels to skip some of them, or at higher levels to help with the final push to 20, or once in epic levels to boost progress towards another destiny.