Changing cassette advice please

I’d say you probably won’t need to adjust the indexing - I never have after changing a cassette.
The chain length might be an issue but I would just go ahead and fit the new cassette and see if it works.
To check the chain length you should put the chain around the biggest chainring and the biggest sprocket on the cassette without going around the derailleur and you should have 2 full links overlap. With this slightly larger cassette you will have slightly less overlap but it should be ok
You might have to adjust the B screw to move the derailleur away from the chain when it’s on the biggest sprocket, but you’ll see this once it’s fitted. GCN and parktools have good videos on YouTube for these adjustments

Road.cc had a Shimano oriented article a while back about getting low gearing for gravel bikes They go through a fairly lengthy list of what will work and what won't

Eamonnator wrote: »

48/31 seems to be a big difference, for your chainrings, do you have any problems changing from big ring to small ring?

I'm currently running 48/32 rings without problem...

Shimano mechs have a stated maximum sprocket size, which is usually very conservative. However there is a limit - for example, if it is a road, short-cage mech with a max of 30T, you can usually get away with a 32T but a 34T might be a step too far. Your options then are either a "Wolftooth" derailleur mounting bracket or one of the new Shimano RX or GRX mechs.

smacl wrote: »

Ultegra RD-R8000-GS and 105 RD-R7000-GS medium cage are specced at 28T - 34T, see https://bike.shimano.com/en-EU/product/component/ultegra-r8000/RD-R8000-GS.html If the OP is already at 32T and the bike isn't too old, good possibility it is one of these and existing rear mech will be ok. OP is unlikely to be on short cage as that is only specced to 30T AFAIK.

True, but current 30T road mechs will happily run out to at least 32T - I'm using one. It gets a bit iffy at 34T

smacl wrote: »

Any idea if the medium cage would run 36T? Tempted by a GRX mech for the cross bike as I'm currently off and pushing on steeper off road sections.

All the new medium cage (GS) mechs are quoted to 34T -any of the older GS versions eg 105 5800 series are specced to 32T.

For the newer kit, I'd have zero concerns about pushing it from 34T to 36T, but I haven't tried it myself.

There used be a nice trick with the compatibility of the Shimano road and MTB kit where you could run a 9 speed MTB mech with 10 speed STI road levers and the MTB mechs were cheaper and easier to get with good capacities. However, that went out the window with the move to 11 speed. (I guess you could run a 10 speed MTB mech with 11 speed road levers using one of those Travel Agent yokes but I think the accumulation of dust, grit and general crap off-road will soon play havoc with the shifting)

Macy0161 wrote: »

Cheaper options are available. I used one from aliexpress for over a year (before I changed the derailluer)

https://www.aliexpress.com/item/32808632946.html?spm=a2g0s.9042311.0.0.27424c4dXVwbUH

Worth pointing out that all a Woftooth (or other version) will do is mount the derailleur lower, allowing a larger sprocket to be used. It does not give the derailleur any more chain capacity