If I leave an alliance during ROC do I loose my progress?

You will lose your season contribution score that is with the alliance. Joining a new alliance will let you start earning this again. You will need to reach at least 10,000 alliance season score to qualify for the end of season rewards.

Your territories will remain and your own influence score will remain.

Your honor buildings will be recalled and you will need to join a new alliance to be able to place them again. Until then you wont be able to process resources and you will lose the benefits from the other buildings.

There is usually a 24 hour timeout from leaving one alliance to being able to join another one during RoC.