I joined their 10 days tour to Australia in 2004 and the service was good. The tour guide was very experience and take care of us quite well.
In your case, they requested you transfering to another tour probably due to 2 reasons:
1) the tour you want to book is fulled
2) not enough people to join so they try to combine people into one tour
You can go or not depends on whether they have sufficient clients to join that tour. If not they might cancel. You can check with them from time to time to see whether the tour is confirmed.
Well, Sunflower not the biggest travel agency but still I think is well established. I do have good or bad experience with Hongtai and Kwankin before, so it really depends.