Today I was practicing a GPS approach and had been cleared for the
approach with the normal, "Cessna XXX, cross ELESE at 3,000, cleared
GPS 15 approach."

A minute or so later the controllers switched positions and another one
took over that slice of airspace.

The new one came on frequency and called my aircraft with, "Cessna XXX,
traffic one o'clock, 2,500 and two miles, southbound" (the traffic was
was crossing my path right to left underneath me).

I replied, "Negative traffic" to which he responded, "Maintain 3,000."

Being momentarily confused, I called to clarify the altitude
restriction. The controller responded rather tersely that he wanted me
at 3,000 for traffic avoidance.

Should the controller have canceled my approach clearance first, then
issued the altitude restriction?

I was initially confused because I still had 5 miles at 3,000 feet
before stepping down to the next altitude as part of the approach, and
it seemed that his first call was simply reinforcing the altitude
minimums on the approach (that is, until he responded in a terse manner
that he wanted to keep me there without ever rescinding my approach
clearance).

--
Peter