chdi is informal, more similar to ti than chi in usage. But it can't be used everywhere.
I'm not sure of the rules where chdi can be used instead of ti. ("Where it sounds right" isn't very helpful.)
It feels safe to use it as an object (Mae o'n licio chdi) or after a preposition (efo chdi), or when it's standing alone as in chdi sy'n iawn. But not as a subject.