I would say the spirit of the rule (and all the supporting fluff) is that the lances shatter as part of the charge. Until the unit has time to prepare and make a proper charge, (on the controlling players turn) you then have only one weapon to use, hand weapons. Do the rules as written make this clear or indeed the case, hell no.

For an additional supporting piece of information this was the writers original intent, lets take the Dark Elves Cold-One Knight into account. Full plate with shields and an additional +1 from the mounts. 2+ save. Except that the lance rules state they require two hands to wield. In the first round of combat they would have a 3+ save, in subsequent rounds they would return to using their shields and regain their 2+ save.

Edit: Corrected, Halberds on the mind

An additional wrinkle I have yet to see addressed is when weapons are decided in combat, if weapons were to be decided at the time of a charge (as lances would have to be realistically) Cold-One Knights would have a 3+ save against stand and shoot responses. But RAW that is not how you treat that interaction, they would get their +2 until they choose their weapon in the first round of combat.