If Nintendo makes a "pro" version, then certainly not before 2021.
The main problem however is to make it work, meaning giving the "pro"console enough power to visually make a difference to the base model without draining the batteries too fast. To pull that off, the Switch pro would need something quite a bit more powerful than an Tegra X2, but something like Xavier on the other hand is consuming way too much to be viable.
A Switch Pro would need to double the GPU processing power and increase the CPU processing power by quite a bit, too, to make a visible visual improvement in most games. While that's possible to achieve right now (Snapdragon 855 could theoretically, though not sure if it can sustain that power, leaving just the 8cx as a chip to pull it off, but that one already comes damn close to laptop consumption rates - too much for a handheld), it's still cutting edge technology and thus way too pricey for a console. I mean, either of the two chips alone probably sells for about the price of a Switch.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454