Praise be to Allah.
The intention is not essential for it to be permissible to wipe over the khufoof. So long as the individual put on the khufoof when he was in a state of purity, he may wipe over them. It is not essential for him to form the intention to wipe over them before doing wudoo’; rather when he reaches that point in his wudoo’, he may wipe over them and that is sufficient.
Shaykh Ibn ‘Uthaymeen was asked: is it essential in order for it to be permissible to wipe over the khufoof that one should form the intention to wipe over them?
He replied: Forming the intention in this case is not obligatory, because this is a matter the ruling on which is connected to whether it is present or not, so there is no need for the intention. That is similar to the case when one wears a thobe: it is not necessary to form the intention to cover one’s ‘awrah with it when praying, for example. So in the case of wearing khufoof, it is not essential to form the intention to wipe over them. The same applies to the intention with regard to the length of time. If a person is travelling he may wipe over them for three days, whether he intends that or not; and if he is not travelling he may wipe over them for one day and one night, whether he intends that or not.
End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen, 11/117
With regard to the one who wipes over his khufoof then remembers that the period for wiping over them has ended, if that is immediately after wiping over them, as the questioner mentioned, then the way to proceed is to take off the khufoof and wash the feet only, because in that case there will still be continuity in washing the parts of the body to be washed in wudoo’, and there will be no lengthy interruption between wiping the head and washing the feet.
But if he remembers that a long time after finishing wudoo’, then he has to repeat wudoo’ and wash his feet, because with this lengthy interruption it cannot be regarded as a continuation of wudoo’.
See: ash-Sharh al-Mumti‘, 1/355
And Allah knows best.