Hi There!
In my honest opinion you can do it whenever you feel like it, reason being there is no change in the moves Onix/Steelix learn both of them learn exactly the same moves at the sames level.
Lv.
1 Bind NORMAL Physical 15 85
1 Fire Fang FIRE Physical 65 95
1 Ice Fang ICE Physical 65 95
1 Mud Sport GROUND Status - -
1 Tackle NORMAL Physical 50 100
1 Thunder Fang ELECTRIC Physical 65 95
1 Harden NORMAL Status - -
4 Curse GHOST Status - -
7 Rock Throw ROCK Physical 50 90
10 Rock Tomb ROCK Physical 60 95
13 Rage NORMAL Physical 20 100
16 Stealth Rock ROCK Status - -
19 Autotomize STEEL Status - -
20 Gyro Ball STEEL Physical - 100
22 Smack Down ROCK Physical 50 100
25 Dragon Breath DRAGON Special 60 100
28 Slam NORMAL Physical 80 75
31 Screech NORMAL Status - 85
34 Rock Slide ROCK Physical 75 90
37 Crunch DARK Physical 80 100
40 Iron Tail STEEL Physical 100 75
43 Dig GROUND Physical 80 100
46 Stone Edge ROCK Physical 100 80
49 Double-Edge NORMAL Physical 120 100
52 Sandstorm ROCK Status - -
But once again only my opinion, someone might know something i dont lol.
There might be one/two additional moves steelix learns but thats the only difference - 37 Crunch, 19 Autotomize
Have fun