I recently had to do this and figured out how to convert the Search Terms into all lowercase.
As Simone says, you need to create a custom filter. Note: The order of your filters is very important.
I set up my "Search Term Filter" by using Custom > Advanced >
- Field A -> Extract A (Request URI) [search URI pattern]
- Output To -> Constructor (Search Term) [$A1]
List of Filter in Order:
1. Lowercase Request URI
2. Search Term Filter
As data is coming into GA, the request URI is going to be lowercased. Then it passes those results to the next filter, which takes in the request URI (now lowercased) and output into a clean search term.
Now someone searching for 'banana' and 'BANANA' will count as one.
I guess in theory, you could instead create a lowercase filter for "Search Term" as your Filter Field, but you would have to place this filter after the "Search Term Filter" because the output of the Search Term Filter is a Search Term (not request URI). I have not tried this though. Basically, you can either lowercase your Request URI first then pass it through the Search Term Filter; or you can filter your Search Terms with all different cases, then pass it through a filter to make them all lowercase. Just be careful, whichever you choose, take note of the input/output data and order the lowercase filter accordingly.
Also, it takes a while before the filter actually kicks in (like an hour or so), so you can't see it work immediately. Sometimes the 'verify this filter' link works, sometimes it doesn't. Also, it's not retroactive so it doesn't convert data collected previous to the lowercase filter; only going forward. Took me a while to figure this out and thought I was doing something wrong.