There really isn't a reason to change it off single rail.
^^^This^^^
The main reason there are multi rail PSU nowadays is because there are some restrictions with regards to how much current can be passed through a single conductor. So manufactures got around that by increasing the number of rails.
Sorry, but that is not, and never was true. That is not how electricity works. The current through any circuit is determined by the demand the load puts on the supply. If the demand on the 12V circuit is 300W, the PSU will deliver 12 volts at 25 amps (12V x 25A = 300W). It does not matter if the supply is a 500W supply or a 1000W supply.
So it is then up to the motherboard designer to ensure any demands through any motherboard circuit do not exceed the ATX standards. Note for graphics cards, the PCIe standard allows for a maximum of 75W through the PCIe slot. But we all know many cards demand much more than that so they get their additional power via separate power cables directly from the PSU.
It is not about restrictions on current that can be passed through a conductor. Nor is it about manufacturers getting around those restrictions (except graphics card makers). In fact, multiple rails imposes restrictions! Why? If you have a supply that is rated for 500W at 12V, and that 12V is split between two rails, then the maximum the supply can deliver on any one rail is 250W! What if your monster graphics card needs 300W. You will be out of luck because in effect, you just have two small 250W PSUs when what you need is a 300+ watt supply.
There are more sophisticated multi-rail supplies that can automatically redistribute some of the available power from one rail to another rail. But they must always leave some in reserve on all rails. And note those supplies are also more expensive and more complex with more components that might fail.
So in multi-rail supplies, there will always be restrictions preventing delivery of the full capability of the supply.
In a single rail, full power is always available.
Multiple rail PSUs
for computers really are just a marketing gimmick. In other electronics, where power "
isolation" is desired for "fault tolerance" scenarios (where you don't want a failure in one connected device to take down another) multi-rail supplies may be desired. In a data center, for example, you might have an equipment rack populated with 10 server computers, all powered from a single power supply. You don't want a fault in the power circuit for one computer to take out the remaining nine servers. So that PSU would be multi-rail with each rail being isolated from the other.
In a single PC with a multi-rail PSU, if you lose the 12V to your motherboard, does it really matter if you still got 12V going to the graphics card? No. To your fans or drives? No.
There is really no advantage or disadvantage.
Ummm, sorry, but also not true. As I noted above, a major disadvantage is some of the PSU's capability must be reserved for each rail - even if that rail is unused. Multi-rail PSUs are more complex, thus they have more potential failure points, and they cost more. On a single rail system, all the power is available. They use a simpler design for more reliability and lower costs.