Hello Coltrane,
The 47k resistor is in the circuit to keep the grid of the 12AT7 at DC ground potential. If this resistor gets too high in value, there will be sufficient current flowing through this resistor to develop a negative grid voltage (the Tode does this with its 10M grid leak resistor).
For a 12AT7, anything up to 500k, maybe even 750k is going to work just fine.
On the other end, if that resistor drops too low in value, it will load the stage before it, and you will end up with some frequency response issues. Any of the Bottlehead preamps are going to work pretty well into a 15K load with relatively short cables, and 47k is plenty high. For certain triodes prone to stability issues, having a low value grid leak resistor is a good idea (the old type 50 comes to mind).