I would hold on to him,30 isn't that old. So he can still manage to help your team win. Plus that gives your younger player time to develop. Depending on how much you trust your 30 yr old C. I would hold on to him until he's 33. By then your young star should be ready to take over the starting position. 32, if he's ahead of schedule. Granted you probably wont get much back for your C at the age of 33 as you would at 30.
On my team it took Lee three seasons until he was a starter. I tried him out first as a starter and early on he was doing amazing but after five games in he couldn't buy a shot. So i know waiting three seasons n training him up works. And i decided to lose some money since i held on to my older center at that time.
Also my starting PG is 30, i have younger PGS Ed and Smallwood. But im gunna hold on to my 30 yr old pg after this season bc what he brings to my team. So maybe its the same with your 30 yr old C, idk how you feel about him, if he's declining or what. Only you would know that.
Really there is no wrong way to go. Whatever you feel is best for your club. Are you a contender now or in the next two seasons? If so, maybe hold on to him.
If not, maybe get as much $ as u can for him and start a youngster at C (18-20 yr old)
These are all things i consider before making a move. That plus the league im in. Strong/weak league.
And those are all my secrets lol