i need your help. when it comes to mechanics of the bike, i'm still a noob.

currently on my bike i'm running full ultegra 9 speed components. since i've injured my knee, doc said i can still ride, but need to take it easy.

so i'm thinking about changing out my ultegra 9 speed cassette 11/23 to a SRAM 9-speed cassette 11/34

my guess is that besides this change, i'd probably need to change out the chain and possible the shifter cable.

but the main question remains is, would i be able to use this SRAM with all of my other ultegra group?

please help.. thanks.