unless you wanna risk running low on gearbox oil or having to go out and buy new oil, yes I'd advise draining & store your current gear oil for later IF it's in good condition. if it's all very dark n old, imo renew it.
depending on how familiar you are with the micra engine bay & basic tools, to me it's real easy.
- lower car down onto a level ground for measuring oil level later..
- remove airbox to allow access down into where the speedo cable enters the back of the gearbox.
- unscrew speedo cable off the gearbox.
- remove 10mm bolt holding the speedo pinion unit to the gearbox.
- remove the speedo pinion unit by twisting the bolt hole tab anti-clockwise to one side then gently pull & twist the tab upwards to remove it.
- stick a long pipe & funnel down into the hole.
- pour bout 2.5-3L of gear oil in bit by bit like engine oil, dont wanna overfill.
- on 1.3 manual boxes, check the gear oil level by sticking the engine oil dipstick down the hole to the bottom and check marking (the dipstick max level measures the same as what the spec says).
- fit & bolt in speedo unit.
- align the little notch in the speedo inner cable to the receiving gap in the speedo units hole & insert & screw the outer cable down.
- fit airbox.