General :: Sed - Change And Capitalize Some Letters In Words?

I'm trying to use Sed to change all "l" letters with "I" in capitalized words something like this:

1. ONlON -> ONION

- and as opposite change the all "I" with "l" in a word in small letters such as:

2. DiIbert -> Dilbert

For the first case I'm using the below sentence which unfortunately changes words such as "let" in "It"

Code:

sed 's/[:upper:]*l[:upper:]*/I/g' test.srt

For the second case the below command seems to work but it might have a side effect as the one used in the first case:

Code:

sed 's/[:lower:]*I[:lower:]*/l/g' test.srt