After reading through the entire topic here, I've come to the following conclusion (keep in mind that I am in no way a professional):
- it is possible to change a mobile phone's IMEI number
- if you have another handset, that is not grey- or blacklisted, you can use the IMEI number of this phone during this process
The questions that remains open for me are:
- do I need any kind of special equipment to change the IMEI number or connecting the handset to my computer with a data cable will be enough?
- can I transfer the IMEI number between different brands? eg. I have a 2-3 yrs old Samsung handset that I'm not using anymore, can I transfer its IMEI to a Nokia or other brands?