Is it worth calling to change? If you are sure you won't fly from DPS, of course, as you're on a Flex Plus there won't be a change fee, but you will have to pay a fare difference if there is one (which there probably is, and in Flex Plus it might be significant as Flex Plus pricing tends to vary more by market than Flex in my experience). Emirates won't waive any fees unless an actual disruption occurs, however if there is a disruption typically they shouldn't have an issue changing your point of departure for the return without any fee.

One issue if you end up cancelling is how EK calculates the refund. If you cancel now (pre-eruption) they might argue that your ticket should be repriced as a one-way which will probably be higher than the Saver componen of JFK-DXB-DEL as you saw it when you originally booked the tickets, and in my experience EK won't confirm the refund amount until after you've made the request. Best case either way is probably to wait and see if there is in fact a disruption.

With several weeks to go and talk of a "possible" eruption I'd say just wait and see. Also check that your trip insurance covers disruption from volcanic eruption, IIRC it's listed as an exclusion/force majeur by some companies.

Also if you change your return from DPS you may (probably will) lose your upgrades to J, unless there is current availability for 3 (guessing 2 adults + toddler >2yrs + lap infant)