I asked for a totally different question, you simply can asnwer to me it will cause issue, It wont cause issue, or I dont know. anyway ill take the third option.
Please close this thread.
I already answered you, and so did Matt. There will be issues and we won't be able to support you fully because the 1.6.5 file isn't meant to be ran on 1.6.4.
If you do just replace the file with the 1.6.5 version, please don't be surprised when we just link you back here when you make more threads complaining of having problems with your forum or having bugs that have been fixed in 1.6.5. If you ask a question like this we won't necessarily just answer your question straight, we advise the best option for you to take. If you do not want to take that advice, that is up to you.