Find and replace with mixed formatting

[pogonoforysci1]

Can anyone tell me how can I find and replace a phrase with another version which has the same text but different formatting (like OH to OH with the H in subscript) It is extremely important to me. At the moment I can replace it with both O and H in subscript but not with H in subscript only. I tried to just do find and change the highlighted bits manually but the highlight disappears when I paste into the first item that was highlighted. THIS IS EXTREMELY AGGRAVATING!!! is there a way to do find and then select all found items so I could replace them in one go by doing paste???? I will be very grateful for any help