The other sees Northampton hooker Mike Haywood replace Billy Vunipola while Armitage takes the place of Irish teammate Marcus Watson.
There are plenty of other familiar names in the squad including Owen Farrell - son of England cross-codes star Andy Farrell - and English-qualified Manu Tuilagli who is the youngest of the Samoan Tuilagi brothers.
And Hunter is backing both Armitage and Haywood to make a big impact in England's Under-20 Six Nations campaign which kicks off against Wales in Llanelli.
He said: "I've been impressed with both Guy and Mike, they came out to Portugal with us on our training camp and looked sharp.
"They were close to being named in the initial squad but there's a lot of competition at this age grade and a lot of players getting first team rugby at their respective clubs, so it's tough to narrow that down to a squad of 32.
"We've also got a few players coming back from injury and that will strengthen us hugely.
"Elliot Daly is doing very well with Wasps while Owen Farrell and Manu Tuilagi are seeing a lot of action with their clubs and the likes of Will Collier, Rob Buchanan and Ben Ransom have been on the bench for Premiership matches - that kind of experience is vital in their development.
"It's important to take each 6 Nations game one at a time, we start off by playing Wales in Wales and that will be tough but we've got a good squad and I'm confident we can go there and give them a good game."