Jed Lamb has opened up about how hard it was to tell the Sydney Swans he was leaving, but says he couldn't refuse a "once in a lifetime" opportunity to join Greater Western Sydney.

Following his decision to shift across town, 21-year-old Lamb is looming as a key figure in the growing rivalry between the two Sydney teams.

He has already been warned by some ex-teammates to expect plenty of stick when the clubs meet at GIANTS Stadium in round one next year.

The Swans offered him a two-year deal early last season, before he made his AFL debut, and coach John Longmire made regular contact to remind Lamb he was a required player.

But after 12 senior games, Lamb found himself slipping down the pecking order late in the year and he didn't feature in their finals campaign.

In a wide-ranging interview with AFL.com.au following his first official GWS training session, Lamb said a three-year deal and the promise of regular senior football was too good to turn down.

"The contract the Swans put together was all right, but my manager said to hold off and see what happens," Lamb said on Monday.

"It wasn't until the second week of the trade period that my manager gave me a call and said 'GWS have offered you a really good contract'.

"There are still a lot of injured blokes to come back at the Swans and with Buddy Franklin coming in, it's a different position, but he's another player that's got to play each week.

"Towards the end of the year a few boys jumped in front of me and I was pretty disappointed not to play finals, but I guess that's footy.

"I had a chat to my manager and my family and thought the best thing for me was to come here and try to play a lot more senior footy."

Once he made his mind up, the next step was telling the Swans.

Lamb was drafted one month after his 18th birthday and, after some early homesickness, had formed some close friendships at the club.

"Once I made my decision, I flew back to Sydney to have a chat with (Swans football manager) Dean Moore and let him know it was a once-in-a-lifetime opportunity and too good to knock back," he said.

"Then I rang Horse (Longmire) and messaged a few of the leaders and let them and my close mates know.

"I was really nervous at first, but Jude (Bolton) and all those guys just said 'If you ever need anything or want to catch up with for a coffee, let me know'.

"That made it a lot easier and once I told the club it was a weight off my shoulders."

While fellow recruits Shane Mumford and Heath Shaw have been introducing themselves to their new GWS teammates at training, Lamb already knows some of them.

Lamb played junior football with Devon Smith, Tom Bugg and Adam Treloar and crossed paths with Jeremy Cameron.

"I actually played under 18s with him at Vic Country," fellow small forward Smith said on Monday.

"I think he kicked four or five goals that day against Vic Metro and I was bottom age, 16 or 17.

"I'm rapt to have him on board."

Lamb's best mate Gary Rohan was shattered by his decision, but the former Swan assured him he'll "still be at your house every night annoying you."

Carlton, the Western Bulldogs, Essendon and the Brisbane Lions were all interested in Lamb during the trade period, so Rohan can at least comfort himself with the fact his mate is staying in Sydney.

Lamb is planning to move into an apartment with another Swan Tommy Walsh in Coogee and see how he finds the commute to Homebush Bay.

He is also getting accustomed to his new colours and is relishing the prospect of his first clash with his former teammates.

"It feels good," he said. "I was used to red, but the family is going to have to change to the orange now. Hopefully it suits me.

"I went to catch up with my old house mates (and young Swans) Tim Membrey and Jake Lloyd and they were getting stuck into me, telling me all the boys are going to give it to me if I'm playing in round one.

"No doubt I'll cop it a bit, but I'm looking forward to it.

"Hopefully the rivalry grows. It'll be good for footy in Sydney I reckon."