B757/767 will adjust to new datum in ALT HOLD or level in VNAV PATH if the subscale is changed. However, our SOPs (and most others I believe) say: Select new level or alt on MCP, then change subscale, then initiate climb or descent. Any other order, if you were to get distracted after iniating climb or descent and forget to reset the subscale then you're going to get a level bust. The transitory climb or descent as you reset the subscale is not noticeable as it takes at least 30secs for the jet to start climbing or descending and by then you've initiated the required mode.