If you are wanting to leave just the league, then that would be easy. I think 8 years is a long time in the same league. Sometimes a change of venue or competition revives you. You can ask your team if they want to join you but I have a feeling you want to.....
Cutting all ties from league AND team. I think you made a great point in your description. Just explain to them you want to change things up and bowl in a different league that is more competive. Making the excuse all nice and fluffy doesn't mask the fact you want to move on. So just be honest, that is all I would want if I was your teammate. As for the league, they'll get over it and forget about you within a month.
p.s. Forgot to mention, if your teammate is your significant other, then all bets are off. You are on your own.
