Is this an easy task to do, swapping covers so the oil filler is on the other side?
Would I need to replace the gaskets or can I just unscrew swap and screw back on?

Didn't you already ask this? First off are you asking how easy it is to do after it's already in the car or on engine stand? If engine stand then it's like a 1 minute job, just swap them... Gasket is rubber and reusable but if it's still original you want to replace it because it gets flattened over time.

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86 Fiero GT 4spd - (pending L67 swap: VS cam, GenV)
98 GTP - Some mods

Should be gravy unless there are brackets covering the valve covers. Several 3800s use the alternator bracket on the rear side even if the alternator is relocated. This would be the bracket/housing that uses the plastic elbows into the intake manifold. Those elbows should be replaced anytime you move them. The orings get deformed and the elbows are normally plastic(break).This bracket is needed unless the intake was tapped for threaded fittings. Also some have brackets covering the valve covers for engine mounts. You need to look to see what is in the way. If you are asking, I can only assume it is in car and you have never removed 3800 valve covers.
It would most likely be easier to use an extended oil fill tube.
As far as the gaskets, you have to take in account the age of the gaskets. If older than a year, I'd replace them.

It's not in the car it's on the cradle, it's a NA series 3 and the alternator bracket covers a bit on the end, I'll have to see how much it covers.

Not in Fieros, but most of the Series II and III I have done, the Alternator/cooling line bracket has to come off. Just did a lower intake on an 03 Impala 10 days ago. You can take out the bolts and move it out enough to access the covers but you would normally break the seal on the elbows. Then you have to pull it anyway. Better to pull it and do it right then to take a chance to have the elbows leak after you got it all back together. Also there are metal replacement elbows for them. Can't stand the plastic ones. Always seem to break if it had Dexcool in it. They get (stuck) in the aluminum.

Yep remove stuff in way and replace with new gaskets the black ones not the gray or orange gaskets. Dan