Q:

Can a caster use a readied spell to counterspell a lower (less potent) version of the same spell?

I have a scenario that will likely never happen, but I am curious in how it would work.
First, the preliminary, Combining Magical Effects:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

Now the setup:
Bob the 3nd-level Generic Wizard and Doug the 3rd-level Earth Wizard face off. Bob knows Doug's favorite tactic and readies a spell. Doug casts Earth Tremor under Bob. The spell say, "You cause a tremor in the ground within range. Each creature other than you in that area must make a Dexterity saving throw."
But Bob was ready and casts Earth Tremor as a 2nd-level spell for his reaction making it more potent (more damage) in the same area.
So what happens?
Doug's casting means Doug is not targeted by the tremor. But as a reaction Bob casts a more powerful version where Bob is not targeted by the spell. So does that overpower Doug's spell? And if so, does that mean Bob no longer has to make a Dexterity save and Doug does?
There may be other spells that do this but Earth Tremor was the first I found with wording stating that regardless of the target point, the caster is not affected.

A:

It's an interesting question but Earth Tremor has a duration of 'Instantaneous' so, in this specific example, the two spells never overlap so Bob is targeted by the first spell and then his held spell kicks in and Doug is the target.

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.

Page 190 of the PHB also says

If the reaction interrupts another creature’s turn, that creature can continue its turn right after the reaction

which helps reinforce the fact that two instantaneous effects do not occur at the exact same time. Thank to Thomas Markov for pointing this out.
If this scenario happened agin with spells that do have a duration longer than instantaneous then I believe your interpretation would work as you explained. But, as you pointed out, this is so situational as to be very unlikely to ever actually happen.

