Is it possible to upgrade these bulbs or change to h7
No completely different design.
But you can upgrade the bulbs themselves but stick to standard wattage dont be tempted by 80 and 100 watt bulbs yes they will be brighter until your headlights internals and wiring loom start to melt with the excess heat and load.
Another no no is Hid or Led dip bulb set ups both are not road legal and will fail an mot and risk prosecution if caught using on public road.
Your 2 main players in what you need is Phillips and Osram
Either something like Phillips extreme vision or Osram Nightbreaker plus ( what i have ) great upgrade gives a brighter whiter light out of same current draw.
You have to remember by modern car design standard Z3 headlights are mediocre and all you can do with standard units is keep outer lenses clean use osram or similar and thats about as good as your getting on stock units.
Stephen.