I'm looking for a word that describes a generic change in altitude, something that could refer to both ascent or descent.
If you really need a single, attested word, maybe consider climbing ?
Though unadorned climbing is usually an ascent, mountaineers climb up and climb down mountains.
Advantages: short, natural, well-attested, easily-understood word. Useful in non-technical contexts, particularly if room for elaboration or qualification is available.
Disadvantages: implicit bias to ascent without qualification: a weakness in a technical context. Would be misunderstood in aeronautics.