Mechanics of updating plan documents with custodians

I'm looking at a custodian that offers model plan documents for their "individual 401(k) accounts".  They can offer traditional and roth accounts. Since the model documents are a little restrictive in their options/elections we were looking to bring in more customized documents via a TPA.

Something i'm trying to figure out in the mechanics of this updating of document is whether the broker that has custody of the assets (and where the accounts were created) 'needs to know' if we updated the underlying plan documents. The role of the custodian here appears to be simply reporting 1099-R (or generating the data for the reporting of 1099-R). 

The question therefore is, if/when we might implement a new set of plan docs, is there any reason (other than the custodian desiring it) that the custodian not being informed of this might be a problem in terms of plan compliance or tax compliance?  On the surface I can't see anything so am wondering what I might be missing here.

Consider checking your client’s agreement with the custodian and each other service provider.

Many retirement-services providers include in an agreement a customer’s warranties about its plan’s documents.  Many agreements include a customer’s promise not to change any document that governs the plan except with the service provider’s written assent.

Some agreements presume documents furnished by the provider are acceptable to the provider, and impose obligations about “outside” documents.

While these service provisions often might not affect a plan sponsor’s ability to amend its plan, these provisions might affect a service provider’s obligations.

In my experience it is very likely that the documents and the accounts are linked.  I'm not sure the custodian (and are you sure they aren't also the trustee - how are the accounts titled?) would want to be preparing 1099-Rs if it is not their document (however crappy their document is).  We have some clients that want to use our document but invest with Vanguard or Fidelity, and we have to be very clear that when they open their accounts they explain that they are not using the investment company document, otherwise you will almost inevitably get the investment company sending an Adoption Agreement.  The magic words seem to be "non-prototype account" but they will still make every effort to screw things up.  

Good insights, thank you both.  As I mull over this, I think I've raised a second, and related question:

The primary purpose of not updating the custodian agreements is that it might result in a new account number and having to close/reopen accounts (general paperwork headaches).  But, in order to execute the new 401(k) we were thinking to utilize two different 401(k) accounts at the custodian. One would house pre-tax, and the latter would be for after-tax (voluntary contributions) that would be then transferred to Roth.  

Since we already have one 401(k) account for Pretax, might the most efficient method here be to open a new account for the purpose of the after-tax funds (which the custodian would not report 1099-R due to the account type being non-prototype generic retirement trust type accounts).

If I convert to Roth IRA then we don't need to update Roth 401(k) docs.

Seems that this might be a viable option with the least amount of friction. Any further thoughts based on that?

You really need to work with the investment company and the TPA on this.  What I was trying to say earlier was that changing to a non-prototype account, if that is indeed what the investment company would call this, typically involves a different type of registration, so it seems unlikely that you can get away without closing and opening an account.  As far as a separate account for Roth, you really need to talk to the TPA about this.  It's not necessary although it may be desirable.  But the last thing I want as a TPA is to find out after-the-fact about all of this stuff.