Changing airports in Beijing

Elena, you are perfectly allowed to do that. There is no doubt about that.

So, since your total transit time will be less than 24 hours, you will rely on 24 hours visa free transit option.

At your boarding in Bangkok you will just say that you will use 24 hours visa free transit in Beijing.

Upon arrival at PEK airport, you will approach 24/144 hours visa free transit desk where you will apply for temporary entry permit. Nothing special, just a formality. You will present your flight tickets and after a short examination, you will be granted an entry and you will be allowed to exit the airport visa free.

After that, you will spend your time in the way you please and just perform check-in at PKX airport (I assume that you were thinking about PKX - Daxing airport when you wrote PEX) and everything will proceed smoothly with the rest of your fomalities.

Also, you are allowed to use separate tickets and you do not have to fly on a single, through ticket. This is also fine. So, just have in mind that both of these airports are eligible for any type of transit and you do not have to take care about it. Also, if possible, I advise you to print out and have your flight tickets in hard copy, if possible.

Hopefully, until your date of departure the Chinese authorities will abolish a mandatory PCR or antigen test (not older than 48 hours) that is a necessary detail at the time being before boarding a China bound flight.