I am going to take the CWI exam next week, however, I lack few months of experience for the CWI title and I know that I will be CAWI instead (in case I passed the CWI score). So, my question is can I upgrade to CWI after those few months pass or shall I have a retest?
You may upgrade if you score high and then follow that up by logging the amount of time required.
Keep in mind that there are serious limitations on the work a CAWI may perform unsupervised. There are a number of threads on the forum detailing this. Just do a search on CAWI.
If it's only a few months that does not sound like much of a problem.
As stated previously, A few months isn't that bad. When the time comes though the AWS wants an upgrade application, eye exam(If it's been over 7 months since your last one was submitted), Work experience(all 5 years), and $240. I believe that's it.