I have the chance of a 4 string banjo that was converted to lefty. If I convert it back to RH will the nut need changing? 
Converted 4 string
I’ve read your other posting and I would suggest that before you buy a 4 string tenor or plectrum, you check out the repertoire available for these instruments to see if this is the banjo music you want to play.
Banjo, in my experience, is not like a guitar where you can just pick it up and strum a few chords. Well, you can do this but on the banjo it sounds pretty dismal.
If the music you hear in your head is clawhammer style, then you’ll want to look for an open back 5 string. If you hear bluegrass Scruggs style, then you’ll have to learn how to use fingerpicks on a 5 string.
Unless the nut on the left handed banjo was already recut from a right handed one, you would have to recut at least the 3rd and 4th string nut to fit the larger strings when you flipped it over. But check out the type of music, old style jazz, Irish tunes, that folks play on a 4 string before you buy one.
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I’ve read your other posting and I would suggest that before you buy a 4 string tenor or plectrum, you check out the repertoire available for these instruments to see if this is the banjo music you want to play.
Banjo, in my experience, is not like a guitar where you can just pick it up and strum a few chords. Well, you can do this but on the banjo it sounds pretty dismal.
If the music you hear in your head is clawhammer style, then you’ll want to look for an open back 5 string. If you hear bluegrass Scruggs style, then you’ll have to learn how to use fingerpicks on a 5 string.
Unless the nut on the left handed banjo was already recut from a right handed one, you would have to recut at least the 3rd and 4th string nut to fit the larger strings when you flipped it over. But check out the type of music, old style jazz, Irish tunes, that folks play on a 4 string before you buy one.
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Once the nut has been cut will it be OK if the strings are reversed back if you follow my meaning.
And yes, I have decided to go to a 5 string! 
I am in the same situation as you - I have played guitar for a number of years then when I retired last year I decided to branch out into the 5 string banjo. As a beginner I can’t give you any expert advice but if you have played bass with two finger picking, I would say that you should be able to adapt to the banjo fairly easily - all you need now is to add in the thumb. The videos on this site are really fantastic and have helped me enormously. They start out very basic and take you step by step through simple picking patterns with close ups of right and left hands so you can see exactly what is going on. The tab version of the notes is at the bottom so you can follow along and you can print off the corresponding tab. I started playing with a couple of friends on guitar and mandolin/fiddle and they have been impressed with how quickly I have progressed. Now of course I want to buy a better banjo!
— Begin quote from "popston"if you have played bass with two finger picking, I would say that you should be able to adapt to the banjo fairly easily - all you need now is to add in the thumb.[/quote
I tend to agree and it is what I thought but I am finding the string spacing difficult and I used to ‘rest’ the thumb on the E string. 
Also, I actually want to not use picks and without long nails the spacing is even harder.
This banjo thing must be a retirement essential. 
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