Shifting the base of the mast aft changed the balance by moving the center of effort aft. It also changed the relationship between your jib and main - and the slot between the sails by opening the slot up.
All other things being equal - the rest of your rig etc.. - did the move increase or decrease the rake of your mast? IMHO it may have tightened your backstay and pulled excess fullness from your main (especially if its a older main with the baggies).
Any number of factors may have changed your boats balance, certainly shifting the mast base aft would make a significant change. Designers agonize for days over calculations to get it 'right' then hope it all works once the boat is in the water.
Generally weather helm (and heeling) is corrected/reduced by
1. Reduce wind pressure on the main or increase the pressure on the head sail. This can be done by shaping (trimming) the sails and dropping the main to leeward. As the wind increases, shape/position become more critical (s/b flatter).
2. If you can, tighten the backstay. This helps to pull the fullness out of the main and will flatten the head sail. The idea is to flatten the sails and keep the draft forward.
3. Tighten the outhaul to flatten the bottom part of the main. At the same time tighten the Main and Genoa halyard to keep the draft(s) forward. Bottom batten should line up with boom, top with windex, twist at the top will allow wind to spill.
4. Move the Genoa lead aft to flatten the the lower section and twist off the leech.The luff of the main bottom batten should line up with boom, top with windex, twist at the top will allow wind to spill
5. Dropping the boom to leeward (via traveller) after shaping will reduce weather helm.