Only players that have passed the KYC and have at least Tier 1 can join the IDO, and they can only join once per project. You do not need to undergo KYC again for future projects, one time is sufficient.
Users join for their highest Tier as well as for the one(s) below it. For instance, if you are Tier 3, you are eligible to win for Tier 3, 2, and 1.
You will lose your entire Tier after funding for a project with the random allotments, regardless of the Tier you won. However, if the requirements change for later projects, your Tier will be unaffected.