Sorry to be late to the party. My idea for a conversion from 1e to 2e would be to have the mythic oaths be archetypes using the free archetype rule for those archetype only. And with the difference you stick to one instead of the usual rule that you have to take two other feats before you can switch. Though I guess you could take an additional non-mythic archetype using your class levels/skill feats as normal.
An easy way to do it is to give access to appropriate class feats that are a few levels higher.
Then when you hit level 20 you gain you archetype as a class that has 10 levels with its own features. Also an extra proficiency level above legendary call mythical I guess.
Do note that would take a lot of time to design and type out. So it would take a company or people with a lot of time on their hands actually to make this. But you know, one can dream.