No — if you wake up after the Porsi time, you should not take “Porsi pachkan” for that same Porsi, because the vow is meant to be taken before the intended eating time and then followed accordingly. Porsi is a fixed morning timing counted from sunrise, and pachkan/pratyakhyan is a clear vow for what you will observe from that point onward. ( jainknowledge.com)

So if Porsi has already passed, the proper approach is:

• do not take the Porsi vow retroactively, and
• if you wish, take a later vow such as Sadh Porsi or another suitable pachkan according to your sangh/guru’s guidance. ( jainknowledge.com)

In simple words: a vow taken after the time has already passed is not considered valid for that missed Porsi. ( jainknowledge.com)