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I agree. I recently grabbed a P6139A out of Raytheon Technologies (RTX, my day job) e-waste bin and found that it works well with my 400 MHz HP 54504A scope (also an RTX rescue). I was able to adjust the probe to get a nice flat response when connected to the calibrator on the 54504A. Fortunately it also looked flat with the calibrator on my Tek 7904.

So, yes, for most purposes a properly adjusted 500 MHz probe will work with a 400 MHz scope. If you're testing pulses up near the limit (handy rule of thumb is pulse rise/fall time is 0.35/3 dB bandwidth), say 0.9 ns (about 400 MHz 3 dB BW), then no, you won't get an accurate picture unless you use the probe specifically designed for the scope. Also handy to remember 1 ns rise or fall time is equivalent to 350 MHz 3 dB BW, and 1 GHz BW is equivalent to 0.35 ns rise or fall time.

And I'd check the input capacitance of the scope (7 pF in the case of the 54504A) versus the output capacitance of the probe (8 pF in the case of the P6139A), and if they are close, you will probably get good results when you connect the probe to the calibrator and adjust it for flatness. Sometimes you can even find the range of capacitance of the probe. The HP 10430A probes I have with the 54504A scope say "1 Megohm||6.5 pF for 1 Megohm||6-9 pF inputs" on them. Also check to see what probe(s) go with what Tek scopes.

There's more, but it's probably covered in those references Raymond mentioned.

Good luck!

Jim Ford