Can AWCF be utilized for a system in the Production Phase? The systems in question have been fielded for many years and have an established AWCF funding stream, but would a transistion back into Production 'reset' the life-cycle and require any spares to be funded with PA funds?
When it comes to appropriation law and which appropriation is appropriate to use, the acquisition phase that a program resides within (i.e., Engineering and Manufacturing Development (EMD), Production and Deployment, Operations and Support (O&S), etc.) is irrelevant. Instead, the legally appropriate appropriation for the effort being performed must always be used. Often times, a program office that is in one acquisition phase may not have an appropriation associated with another acquisition phase, but that is not a legal justification to use an incorrect appropriation. For example, in the scenario you depict in your question, a system that has been fielded for many years likely has an established Operations and Maintenance, Army (OMA) funding stream, but if the actions being taken to those “systems that have been fielded for many years” is Other Procurement Army (OPA) in nature, then OPA is the correct appropriation to use (regardless of what acquisition phase your program resides within, or what established funding stream your program currently has).
In your case, you state that the Army might, “take systems which are currently fielded and in the O&M phase and declare them back in the Production phase.” By this, I assume you mean they are going to upgrade, refurbish, or retrofit previously fielded units, or perhaps even go back to the production line and produce additional new items (all of which are very common scenarios in DoD acquisition). In any of these cases, the correct appropriation to use does not change based on the acquisition phase that a product resides within (in other words, if the Army is going to do something to the items that is Research, Development, Test and Evaluation (RDTE) in nature, then the appropriate RDTE appropriation should be used; if the Army is going to do something to the items that is Procurement in nature, then the appropriate Procurement appropriation should be used (in your example you state this may be OPA); if the Army is going to do something to the items that is Operations and Maintenance (O&M) in nature, then the appropriate O&M appropriation should be used).
A good depiction of this is the Product Improvement Funding Decision Tree, which as you can see from the below chart, is totally independent of what acquisition phase your program resides within, or what established funding stream your program currently has:
As always, you need to consult and work with both your local comptroller and legal offices to ensure that the details of your particular situation are being legally addressed. This is especially important in the case of fiscal or appropriation law, given their detailed and often complicated nature.