Venting the tub, 90 okay?

[Gdunbar]

I am replacing a shower with a bath/shower. The supply lines will need to be dropped a few feet and I need to install the Tub spout. Currently the vent is right in the way if I were to lower the lines. The vent comes up about 4 feet then takes a 90 degree turn left to the stack. The vent is like 3inch steel/iron. Is there any issue if I were to take the vent down to like a 16 inches, then have it go 90 degree left and onto the main stack? Or even like a Z back up to the original tie into the stack?
PLumbers come tomorrow for the big saw cut in the floor for the relocate of the toilet. Will be messy.Thanks