Re: How to deal with my sempai?
I also do not see where the conflict lies here. While it is true that he started before you, would that make him your sempai if he were less skilled than you? I tend to consider sempai not as a term of time - but rather, an assessment of training time and progression.
Your sempai are those who have trained longer than you, progressed farther than you, and who have a responsibility to teach you. He began as your sempai, however you have gone on to train in Aikido longer than him. Surely you have learned some things that he did not know and he undoubtedly has a little catching up to do. Since you have essentially "passed" him (even if not in skill, but perhaps in knowledge) then you are now his sempai.
Thus - no conflict.