yes u will notice the different when u upgrade from a 2nd gen to a 8th gen Intel, Ryzen overall single core speed are pretty much on par with 2nd/3rd gen Intel but they really gave u a bang in the buck, so i don't really think it's really worth it to jump into that leap yet, though u will notice some improvement here and there (8th gen had better single core speed, whilst Ryzen CPUs are all OCable, so u can exploit their potential)
however, if u still keen on keeping the whole thing (since u had a 32GB DDR3 there, would be a shame to just throw it away), an i7 2700k / even an i7 3770k if ur mobo also supports 3rd gen (better find used though), it'll easily last 2-5 years before any real changes is necessary
the i5 2500k is going to struggle since it is only a 4c/4t CPU, but overall performance u won't notice it unless the games are very demanding in terms of CPU usage [it's 4k though, even on normal games it's gonna strain pretty hard]