Murray Wallace switched to three – previously occupied by James Meredith – when the Lions reported back last week.
Striker Matt Smith signed from QPR on Monday and has been allocated number 10, which was worn by Fred Onyedinma in the 2018-19 campaign.
Goalkeeper Frank Fielding, who completed his move last Thursday, took the number one jersey which had been in the possession of Jordan Archer, released earlier this summer.
Harris said: “Nothing is confirmed with squad numbers so players are available to swap at the moment.
“As we stand Murray is wearing three and Bradders asked for nine. The nine shirt was available – if he wants to wear it because he feels it is the best one or a lucky number, then so be it.
“I want people to be happy and confident in what they do. With Bradders it shows a real confidence and desire. He has come back from injury as quickly as he can. To wear the number nine shirt is a huge statement.
“I’m really looking forward to working with him this year and seeing him at his best.
“It will become more relevant as pre-season unfolds. It’s not until later in July we have to give numbers in and there will be changes from last year.
“Someone like Mahlon [Romeo] was offered the number two but is happy wearing 12 because it has been his number through his Millwall career and he doesn’t want to change. Sometimes it can be because people are superstitious.”
PICTURE BY BRIAN TONKS