No need for a loop.
There are three 'types' of characters that are potentially in your string:
- Upper case characters
- Lower case characters
- Symbols without defined case (comma, period, etc)
I would create an array that encode the type. To do this you need to create an upper case and a lower case version of your string (with upper and lower). If you use a char array instead of a string, that will allow you to do element-wise comparisons.
That is how you can find the position of type 3. You can use a similar tactic to find type 1 and 2. Then you can use logical indexing to replace the appropriate characters.
You can also ignore type 3, since those don't change, so it doesn't matter whether you copy the 'lower case' period or the 'upper case' period.