"We just need to rename" or "We need just to rename" [duplicate]
Solution 1:
The placement of just determines whether your emphasis is on the verb rename or on the noun message.
a) Renaming is the only thing we need to do
b) The message is the only thing that needs to be renamed.
All three of your examples produce meaning a).
If you wanted meaning b) it would be ".... just the message".
Solution 2:
I think any of the options is grammatically correct, but the word just is often used rather informally to mean simply.
If you intend just to be synonymous with only it might be better to write:-
"In case we only need to rename the message..."