If Google has already crawled and indexed your website and its webpages, then you shouldn’t really change your title tags because the webpage will be sandboxed for some time (can be weeks or months). Therefore you will see a drop (could be massive) in rankings. The only excuse you have for changing your title tags is if they were really bad in the first place!
I have changed the title tags on the question part of my site so that the first letter of each word is capitalised. This is not really a big change as the title tag will be exactly the same it was before but will be just more pleasant to the eyes. I found that when searching on google, title tags which have the first letter of each word capitalised are more easy to read, which means that it could draw in more visitors for me. Hence I’m making this change today and if the effects are positive, I will implement it to the rest of the site.