Praise be to Allah.

If a person does ‘umrah and does not shave his head or cut his hair out of ignorance, then he puts on his regular clothes, then when he learns of the ruling he has to take off his regular clothes (and put on the ihram garments), and shave his head or cut his hair, but he does not have to do anything (pay any penalty) for what he did of prohibited actions such as wearing regular clothes or anything else, according to the more correct opinion, because he was ignorant or unaware of the ruling. But he has to repent to Allah, may He be exalted, for falling short in learning what he needed to learn. The one who wants to do an act of worship or engage in a transaction must learn the rulings on it; this comes under the heading of knowledge that is obligatory for him to acquire.

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: What is the ruling on one who forgets to shave his head or cut his hair following ‘umrah and puts on tailored clothes, then he remembers that he did not shave his head or cut his hair?

He replied:

The one who forgets to shave his head or cut his hair following ‘umrah, so he does tawaaf and sa‘i, then he puts on regular clothes before shaving his head or cutting his hair, has to take off his regular clothes when he remembers that (and put on the ihram garments, and shave his head or cut his hair, then put his regular clothes back on. If he cuts his hair or shaved his head whilst wearing his regular clothes, out of ignorance or because he forgot, then he does not have to do anything, and that is acceptable; he does not have to repeat cutting the hair or shaving the head. But when he realises, what he must do is change out of his regular clothes so that he can shave his head or cut his hair whilst he is still in ihram.

End quote from Fataawa ash-Shaykh Ibn Baaz (17/436)

And Allah knows best.