No reason to take Kelvin out, so leave him in, I know he is getting on but he is still performing well and thats all that counts.
If Kelvin starts to look a liability Bart is an able deputy, If he then proves to be better than Kelvin then let him keep the role on the same basis.
I do think that one way or another we will need another new keeper within the next 12 months to replace either Kelvin (honourable discharge) or Bart (off to further career).