Oracle Database Discussions

Hello ,
I am working on oracle 11g R2 on AIX.
One query was performing good around 20 sec. but suddenly it took more then 15 min.
We check that the sql executoin plan changes , it showing that order of operation changed like order of using indexes is different.
Now the new plan is not good.
we want to force the old plan of sql to use in future.
I read about sql plan management , it shows a manual method to create baseline and evolve the all plan. In one texample we found that
first query execution plan was created using with out index and then with index So, second plan was good and accepted.
But in this case we do not need to change any thing ,query is performing bad may be becasue changes order of operation ..
One other way to use hint , but for this we need to change sqls , which is not possiable in production now.
The issue is
For this we need to run the sql again and oracle may not create plan like old one.So we will not be having old good plan to accept.
All 2 execution plan are already in cache .

I am looking for a way using that we can set sql plan hash value ( of good plan) or any other id of that sql plan to force to use that plan only.

any idea how to do it ..