Henry De Moor sent in this response to your question:

Technically, the Church Order of the CRCNA insists on having the Form
of Subscription signed by those being ordained at times stipulated by
the council and the classis (Art. 5).  For the council, in my
experience, most people sign it each time they are ordained, but there
are councils that do it only upon the ordination in the first term,
then not in additional terms.  As for the classis, most of them
stipulate that it be done by first-time delegates.  That follows a
more "Presbyterian" way of doing it since they believe in once
ordained, always ordained, and do not tie the ordination to the actual
times of service.  Interestingly, some classes ask a candidate for
ministry who has just passed the ordination exam to sign the form
right there and then, not even waiting until the first time he or she
might be delegated to the classis.  Currently, delegates to synod do
not sign the Form but "stand" for the Public Declaration.  My
prediction is that this will change to signing the new "Covenant," as
we will now call it, so it seems. 

That has also been the practice within Classis Huron. Only first time delegates (to classis) need to sign the Form of Subscription.

And within local church councils, only first time officebearers -- elders or deacons -- generally sign the Form of Subscription. And once Synod adopts the new Covenant, it seems logical that everybody will need to sign the new document.

It does beg the question: Does one's commitment to the Reformed creeds never change after 30, 40 or 50 years .. if you've been a long time office-bearer? The answer, presumably, is No.

In some ways, it's a moot point.  As long as it includes a statement to the effect that we affirm the Contemporary Testimony, I cannot in conscience sign it.

I do not affirm article 38, or articles 47-54 (revised version of CT), the first because it contradicts the Catechism and the Belgic Confession in its teaching concerning the Lord's Supper and affirms in effect a consubstantiationist view of the sacrament; the rest because they bind us to a political agenda I do not share and, even if I did share it, would believe inappropriate as a condition for holding office in the Church.  In addition, certain statements in those articles undermine the doctrine of total depravity and indicate a solidly unreformed view of the task, capabilities, and role of civil government.

As for your own question, if the CT is affirmed, we would have to sign the "covenant" frequently - the CT is designed to change with the times, so there's no guarantee that what you signed up for the first time is what's on the books the next time.