Yes you can switch, but you would just need to install Nvidia onto your PC assuming you still have space to do so. If you don't plan to use AMD again any time soon then you can delete it if you want but just to be safe i would keep it if i were you just in case the Nvidia card might go bad you'd have the spare AMD to back you up. If you plan to save that old card then be sure to wrap it up and place it in a box and store it away some place safe and sturdy.(you should wrap that old card is air filled plastic or bubble wrap to keep it safe if you intend on keeping it)
For the card i suggest a GTX 1060, you can find some on walmart and the Nvidia website for a decent price of $230-$300up. this card should consume about 150w-250w so i also suggest that you upgrade your power supply such as this one for only $50 and is 700Watts.(this is also on walmart)
https://www.walmart.com/ip/Thermaltake-Smart-80-PLUS-700W-Power-Supply/47524920
The reason why i suggest that you upgrade your power supply is because depending on the amount and type of case fans you have they may require more power than the graphic's card at times due to either high amount of heat or high amount in impurities in the air flowing into your PC such as dust and other particles. Also your mother board will also drain power to feed the CPU and other components of your PC, Though i'm not rich these are just my suggestion and answer to help you in times of need. Though if you have a question just question back via reply.
(I prefer to order off walmart and legit store companies such as bestbuy, etc. because i know for sure they will deliver unlike ebay, amazon, and other online retailers)