I started riding and racing again in 2012 after a 20 year hiatus. I felt I had enough fitness back to race Beginner after 7 months. Of the two races I entered the first year, I was able to podium my first race (3rd) and get a 6th in another. Not being able to train much and after seeing the times for Sport and the extra distance, I stayed in Beginner my second year. This series was a small field and it was the same 8-13, 50 yrs+ guys. The general talk in the group was that we couldn't handle the extra laps and speed of Sport at our age (lol!). I started seeing the same faster guys take the top 3 each race and I got fed up when they didn't move up the next year. I decided to jump up to Sport after my first Beginner race of my second season. I figured that if I was going to lose to faster guys, it'd be guys who manned up and raced in the proper class. So, I now ended up finishing in the lower half of the Sport field but I was fine with that. Those fast beginner guys who sandbagged are still in Beginner but act sheepish around me knowing I moved up and am completing races they feel they can't (woot!). I then started racing Sport in a new series with bigger fields and faster guys. I now finish in the last 4 places out of 20-30, lol! But, I'm getting stronger and am looking to place better this season.
I guess my rule for moving up is, stay in your group the whole season unless it's obvious you're killing everyone else (like finishing 20-30 minutes ahead and on your 3rd beer), or move up if you finish in the top 5 overall at the end of the season or if you podiumed a lot. Don't move up after doing well in just a couple races--someone has to be in the top. However, I have a buddy who raced Beginner as he'd never raced before and had no idea how he'd do. He won his first race by 5 minutes or so--not a huge gap. But he moved himself up to Sport for the rest of the season and finished 5th overall with never a podium placing. He will probably stay in Sport for the fact he never podiumed. From what you wrote, no one should call you a sandbagger if you win a few or place high in a few. Usually, you're allowed to complete the season in the same cat unless it's obvious you're too fast like I wrote above. I should say that when I moved up, I too was almost demoralized but the others were very encouraging and impressed that I moved up saying more guys should. So if I end up finishing in the top 1/3 each race but never podiuming, I'll stay in Sport. I think you're ok to stay in Sport for another year unless the race organizers make you move, then move up like yiou said, overall win or not.