RF is definitely safer if all other things are equal but when it is causing a significant problem (e.g. travel sickness) then it's not THAT crucial that you should continue over and above all else, especially when the child is over 15 months. (Even more so when the child is over 2).
Yes, crash safety is important, but everyday comfort is important too. And worrying about vomiting or actual vomiting happening is distracting to the driver, which is much more of a problem - it's better not to be in an accident in the first place than to be in the best seat but a higher chance of crashing.
I think it's important to know that RF is safer but I do think some of the RF advocates go to such an extreme point that it is hard to find out information on how to make FF as safe as possible so that is what I try to do.
If you can stretch it out a bit longer e.g. to 18 months, it's worth doing. But 15 months is a good minimum if you are absolutely desperate to change ASAP. (If you have an R129 seat, it's also a legal minimum).
If you have a spin type seat and can switch based on the journey or start out RF and then turn FF if they show signs of sickness, that is worth doing.
When you are FF, it's really really important to check install and harness tightness. This matters a lot more than it does for RF seats. It's probably one of the bigger factors in why RF is safer, because it is very common to get these things wrong.
So first that the seat is installed as tightly as it can be. If it is isofix, wiggle it to see if you can click the isofix arms in any tighter. Play around with the settings on the leg to find the lowest one which is solidly green. If it's seatbelt fitted, really kneel into the seat to get ALL the slack out of the belt. Double check your manual to see if there are any steps which have been missed.
Double check where the 5 point harness straps are - they should usually be level with the child's shoulders. If they are between two slots, slightly higher is better. If it has pads on the harness, double check the correct positioning for them and make sure they always have the right position and that they aren't swapped, inverted or twisted. Tighten the straps so you can just slip your fingers flat under them. Again, the manual can advise.
Usually you take all inserts out of a RF/FF seat to FF - check the manual.
Maximise space in front of the child so push the front seat as far forward as it can go of course keeping it safe for the adult passenger.
Consider whether it's worth swapping the seat for one which is optimised for FF - a lot of the multi stage/combination ones aren't really FF optimised. BUT, this is splitting hairs, the most important thing is all the stuff I wrote above which you can do with the seat you have now. This is just an optional extra if you have the budget to change the seat and it would make you feel more comfortable. I realise that you have likely not long bought this one, so if it would cause a problem to change it then it's likely not worth doing.
Good features in a FF seat:
Further back in the car (e.g. compared with a spin seat)
FF only
Isofix + support leg or top tether (most are top tether, not leg)
One piece shell/construction, fewer moving parts is better.
Airbags (very expensive though - only Cybex Anoris and Maxi Cosi Axissfix Air have them).
Some kind of energy management system e.g. grippy pads, energy absorbing pads, Britax have a thing which makes the isofix connector absorb some of the forward movement.
Brand - Britax, Recaro, Maxi Cosi and Besafe all get good safety for FF. Other brands are mixed or not so much.
R129 safety standard is worth having as it's stricter especially on the forward movement.
Impact shields aren't overall any safer - on average they have roughly the same rate of real-world protection as all forward facing seats. So a good quality 5 point harness seat, from a good brand, with proper installation and correct use of the harness is probably going to give better performance than a shield. OTOH a wrongly used harness or a poor-performing or mis-installed 5-point-harness seat might give worse performance. Shield seats (especially the seatbelt type) OTOH are rarely misused. The regulations for shield seats were tightened up with R129 as well, so it may be worth opting for an R129 seat if a shield appeals to you.