If you have any spans with spaces, or any character other than A-Z, then you would also have to check for that. You can also do it with two passes:
pass 1
search:
<span class="smcpIncise">(.*?)</span>
<span class="smcpIncise">\L\1\E</span>
pass 2
search:
<span class="smcpIncise">(.*?)</span>
replace:
<span class="smcpIncise">\u\1</span>
This two-pass technique can be saved as a "Saved Search" and run together with a single click.
After that you would need to individually check each span for words you wish to be capitalized like proper names, "I", "I've", "I'm", etc. Although you can create individual S/R for a bunch of those.
It would be really nice if we could have some kind of "Sentence case" or "Title Case" functionality in the S/R, but as I understand it, Sigil's flavor of Regex doesn't support that. If you absolutely need to have it, then you could try Calibre Editor which has the ability to do Regex Functions.
edit: You can get a "Sentence case" like effect by modifying Doitsu's search slightly. Adding a space or punctuation capture into the second group works well:
Code:
search:
<span class="smcpIncise">([[:upper:]])([[:upper:][:space:][:punct:]]{2,})</span>
replace:
<span class="smcpIncise">\1\L\2\E</span>
Cheers,