Correct, “a club day” or “the club day”

I agree with Jason that it is a relative word, not a conjunction. However, relative pronouns usually refer to a preceding noun (except for sentential "which").

In this case, it is at least curious that it refers to an adjective. I don't think the antecedent is "inevitable change", but just "inevitable", the relative clause being more or less equivalent to the far less idiomatic:

• If change is inevitable, and inevitable change is, we should then ask...

Within the relative clause, "which" is the subject complement, and "it" (standing for "change") is the subject of the clause.

Another example without a noun will show my point more clearly:

• If you are interested, which you surely are, don't hesitate to ask.

It is clear that "which" refers to the participle "interested".

You should avoid this, unless you are sure.

This kind of use is very specific to particular groups. If you are not part of that group it would sound odd to call a person "brother".

If you are in a social group, and the other people are referring to you as "brother", then you can copy them. Otherwise don't do it.

The mere fact you have come on the internet to ask if it okay to call your friend "brother" shows me that it is not.

By far the best word to use to refer to your friend is their name. This is "chill and neutral". So say "Hey Joe! How are you?" and not "Hey brother! How are you?"

If you want to refer to a person or group of people by their relationship to you use "friend". So say "I'm meeting my friends later" and not "I'm meeting my brothers later."