I know it is required to restart splunkweb/splunkd after installing Enterprise license first time.
and I don't need to restart splunkweb/splunkd after installing additional Enterprise license (Stacking)
My question is, does adding upgrade license require splunkweb/splunkd be restarted?
I'm not sure I understand what you mean by an "upgrade license." Which version(s) of Splunk are you talking about?
If you have a license, you can upgrade from one version of the product to another (see instructions in the Installation Manual), which definitely requires stopping and restarting but not strictly because of licensing.
If you need a bigger license, you use the stacking feature, which you mentioned (does not require restart).
If you are upgrading from Splunk Free to Splunk Enterprise, and it is the first enterprise license you are installing, you will need to restart Splunk (see Add a license in the Admin Manual).