I am working on two 6509 and few 4848...I just added a VLAN to my VLAN database and would like to allow it on few trunks without them going down. Last time I did, the trunks went down.
All you have to do is to add the following command
switchport trunk allowed vlan add
The trunk should not go down due to this
Note that if you leave out the "add" modifier you will remove all the currently configured VLANs from the trunk. This will almost certainly take the switch off the net and prevent you from communicating with it.
HTH, rate if it does