Triac substitute for BTA12 600TW?
Generally, yes. You can use a Triac with a heavier rating in an application that was intended for a lower rating. Devices with heavier ratings are generally more expensive, that is why devices with lower ratings exist.

There may be some complications:


Heavier devices may have larger parasitic capacitances.
Heavier devices may have a different resistance.
Obviously, the form-factor may be different.
The turn-on current Ig may be different (higher).


However, in most applications this will not be a problem, especially when using devices from the same series, as you are suggesting.