Change does appear to be a synonym of evolve, so yes, ever-evolving and ever-changing mean pretty much the same exact thing.
However, in most any biological cases, evolving is considered for the better, an improvement. But when it comes to things other than biology, say like a workplace evolving, it loses the "for the better" assumption since many times workplaces tend to get worse as time goes on, not better. At least in the employees opinions and experience.
In your first example, it would totally be fine to use ever-changing in place of ever-evolving. It actually would probably make more sense to use ever-changing since slang is seen as a negative by many people, so creating even more slang terms is definitely not "for the better".
In your second example (didn't read the link), ever-evolving is definitely a good choice since it's based on the assumption that the person's definition of success is getting more accurate to what success really, truly means for him. Whether that is making more money or spending more time with friends/family, whatever he feels makes him most successful is "the better for him". So maybe a guy thought making lots of money was what he thought would consider him to be successful, but then later on in life he realized that his relationships with family/friends/etc is what really makes him successful.