So I'm currently looking for 2 J class redemption tickets from CDG>TPE on Oct 23 with a connection in HKG but there's only one seat

The agent said I can redeem a mixed cabin fare in PE/E (since the hkg>tpe leg doesn't have PE) & then reissue to Business J class but she can't see the availability until after i book this redemption? Is this true? I saw someone else on the forum say the agent was able to check if theres seats available for upgrade before booking? I also asked her if there's no space, can she put me on a waitlist & she said they don't have waitlist. is that only for upgrading paid fares with miles?

She also says for both the redemption & reissue pools are different but the 2nd agent said it is the same as what i see online


Do you guys think its better to book the PE flight & then try to get into business class? Is there the 50usd fee to change?
Incase there isn't availability, i can still cancel and rebook the different J flights since she said theres 1 seat for each of the flights?