Is the 'Asr prayer invalidated if the worshiper's intention shifts during the prayer to the Dhuhr prayer, then returns to the 'Asr prayer?

The wandering of thoughts during prayer, and the absence or non-recall of intention, does not invalidate it. What invalidates it is: severing the intention, or resolving to sever it. The proof for this is the hadith of Abu Hurayrah, that the Prophet (peace be upon him) said: «When the call to prayer is made, Satan flees, passing wind... until a man does not know how many rak'ahs he has prayed.»

So, if changing the intention is merely an oversight and not recalling it, this does not invalidate the prayer. However, if it means diverting the intention from the specific prayer to another, this invalidates it, as it constitutes a severing of the intention.

If it is meant that doubt arose during prayer: whether it was, for example, Dhuhr or Asr? Then, if he performed an action with this doubt before the truth became clear to him, this also invalidates it; because a part of his prayer would be devoid of firm intention. However, if he did not perform any action during his doubt until the reality became clear to him, this does not cause harm.