I upgraded very few things from DVD to Blu-Ray--had to get The Twilight Zone and Star Trek TOS. If they ever released THE FUGITIVE I'd get it. Probably DANGER MAN too, but I see no realistic reason that they would.
As for 4k or whatever is next or in the future....how good does it need to look...
I once went out and excitedly bought a DVD set on sale for $99 only to get home and discover I already had it (I'd received it for free and had forgotten). That's when I knew I needed to re-evaluate my DVD mentality.
I was stunned when I had occasion to give some Blu-Ray discs as gifts to family members and discovered NOT ONE person in my family or extended family (and only one of my friends) owned a Blu-Ray player, much less any discs. I knew Blu-rays weren't as successful as DVDs, but had no idea they were...