Neymar, who wore number 11 at Barca as Lionel Messi occupies the coveted slot, joined PSG on Thursday after his release clause of €222million was met. The transfer smashed the previous record set when Manchester United paid €105m to Juventus for Paul Pogba in August 2016.
Javier Pastore took up number 10 at PSG last season when Zlatan Ibrahimovic left for United, but the Argentine has now reverted to the number 27 he wore between 2011 and 2016.
Pastore said: "It is a little gift to say, 'Welcome to Paris'.
"I wanted to offer him this symbolic gift. I want him to feel happy and at home right from day one.
"And I want to offer him the support that he needs so that he and the entire squad can go on and win the Champions League."
Meanwhile, Neymar expressed his delight at joining the Parisians after four successful years at Camp Nou.
"I am extremely happy to join Paris Saint-Germain," said the 25-year-old, who will be presented by his new club on Friday.
"Since I arrived in Europe, the club has always been one of the most competitive and most ambitious. And the biggest challenge, what most motivated me to join my new team-mates is to help the club to conquer the titles that their fans want.
"Paris Saint-Germain's ambition attracted me to the club, along with the passion and the energy this brings. I played four seasons in Europe and I feel ready to take the challenge.
"From today, I will do everything I can to help my new team-mates, to open up new horizons for my club and to bring happiness to its millions of supporters around the world."