Before you can delete an a/c type there must be no log entries using that type. Therefore I suggest clicking the a/c type column header to force a sort on that column, then change the types. However, when you change a type it is going to change the N-Number/Ident so the routine you'll need to use is:
-Click in the Ident column, press CTRL+C to copy the Ident, change the Type, tab, CTRL+V to paste the ident back in.
Before you do anything, backup your data file File..Archive..Backup