To this DM, this situation seems similar enough to the impact of a headband of intellect (DMG (2000) 218). Prior to the 3.5 revision the headband of intellect did not include the text, "This enhancement bonus does not earn the wearer extra skill points when a new level is attained; use the unenhanced Intelligence bonus to determine skill points" (DMG (2003) 258).
Thus, like a 3e creature that wears a headband for the majority of the previous level gains additional skill points when it advances a level, this DM would rule that a 3e creature that spends the majority of the previous level in a different form uses that form's Constitution modifier to determine its hp when it advances a level. This DM would further rule that either benefit is retained even if the effect that granted the benefit is lost.
However, so far as this DM's aware, this majority-of-the-level-vis-a-vis-skill-points ruling is absent from the core rules. This DM believes this ruling to be either long-since-destroyed electronic consensus or, perhaps, an actual ruling by one of the game's representatives, the location of which this DM can't remember. That means it's possible for a DM that goes strictly by the letter of the rules to decree instead that a creature reaps the benefits of donning a headband or, similarly, of assuming a different form possessing a ridiculously high Constitution if the creature does either immediately prior to advancing a level! This player wouldn't expect such generosity, or, were he to benefit from such generosity, he would expect foes to benefit likewise and expect druids, sorcerers, and wizards in particular to possess much higher hp totals than expected even were they, for example, currently in human form.
Note: To be clear, the question's binds seem to limit the answer to using the dnd-3e core rules using only the rules-as-written and online discussions about those core rules—many of which are lost to the æther—, excluding any other sources, even those that might answer this question directly, such as Sage Advice columns from Dragon magazine or the Tome and Blood supplement.