It won't make much of a difference. In fact, if the feedpoint impedance of the antenna is closer to 75 ohms than 50 ohms, it may be better. And if this is much higher quality coax (thus, lower loss) than the alternative, it will certainly be better.
If the coax impedance does not match the load (antenna) impedance, two things happen, which may or may not be a significant problem:
- The VSWR is higher than 1:1, and thus there will be additional losses, and
- the impedance seen by the transmitter will be the load impedance, centered around a constant-VSWR circle centered on 75 ohms.
It's easy to see what this "constant-VSWR circle" on a Smith chart.
Or to put it mathematically, VSWR is defined by:
$$ \left(\text{load impedance} \over \text{coax impedance}\right)^{\pm 1} $$
±1 is selected to give whichever provides a value greater than 1. So let's say the load impedance is 50 ohms, and you are using 75 ohm coax. That's an SWR of 1.5, because $75/50 = 1.5$. Then depending on the feedline length, the transmitter might see any impedance that satisfies:
$$ \left(\text{impedance seen by transmitter} \over 75\right)^{\pm 1} = 1.5 $$
So it might be 50 ohms, or 112.5 ohms, or any number of complex impedances in between.
Is this a problem?
Well, was your load matched to 50 ohms in the first place? End-fed antennas are notoriously difficult to predict, so probably not. So the impedance gets rotated about whether you use 50 ohm or 75 ohm coax, but you probably have a tuner which will be adjusted so the transmitter sees 50 ohms regardless.
Even if you don't have a tuner, a 112.5 ohm load isn't that bad for most transmitters. You'd have to consult the transmitter specifications to see the range of permissible impedances for it to deliver its rated power, but in many cases you'll be fine.