It's true that in Section IX both a change from spray to pulsed spray and the omission of backing are both non-essential variables per QW-255. So the simple answer is that yes, from the information you've provided and with all other things being equal, you should not have to requalify. The complex answer is that all other things are unlikely to remain equal. Here are some things to think about if you haven't already. Keep in mind I don't have Section IX in front of me so any or all of the following may be a figment of my imagination...
That you are using backing leads me to believe that the final weldment is to be CJP from one side only. So with the omission of backing it's likely you'll have an open root weld.
If you plan to GMAW-S the root gap you must requalify the procedure due to change in transfer mode. Also keep in mind GMAW-S is considered a separate process and will affect your welder qualifications, calculation of thickness deposited, thickness qualified, etc.
Deletion of backing is an essential variable for welder performance. If they are not already qualified for the open root you must requalify the welders.
What kind of metal and product form are you welding? Will it require backing gas? If so, you must requalify the procedure.
Is impact testing involved? No doubt you will have different heat input levels with the pulsed spray and may have to requalify the procedure.
Why was pulsing explicitly not permitted in the WPS in the first place? Did you inherit the procedure from the previous welding engineer? Get to the bottom of these things before you follow through so you don't repeat mistakes that somebody else had to learn the hard way.
Noel