In using the "switchport trunk allowed vlan 1,2,3" command, to add a vlan to the existing list of vlans allowed, will I cause spanning tree to re-converge?
I'm pretty sure it wont but I have no way to lab this before a change control that I'd like to perform remotely.
you can use
switchport trunk allowed vlan add newvlan
otherwise you need to rebuild the complete vlan list (including the new one)
And this should start a new instance for newvlan if using PVST+ or RPVST.
existing STP instances are not affected
With MST if the newvlan is already associated with the intended MST instance could even cause no change at all if already the MST instance is in forwarding state over the trunk.
Hope to help
The switches will start to send and process BPDU's for the new vlan so as long as you haven't messed with pri / costs settings per vlan then no and even if you have, it would only affect the new vlan ( I'm assuming your using PVST+ / rapid or MST )