Hi
The only solution is to change the recon account manually through the below steps 1 & 2 proposed by you:
1. I will post adjustment entry for the service vendor1 & 2 so balance will be zero and will change the recon.account to 100001
2. will upload open items into service vendor1 & 2
I do not see any issue in this case. The only thing is make sure, you also clear the open items on Vendor 1 & 2 in step 1 before proceeding to Step 2.
Thanks & Regards
Sanil Bhandari