I'm wanted to upgrade my handset a Nokia 7110e , keep my number and
stay with Orange.....

Orange wanted £85 for the new handset. I've been with them 5 years, I
threatened to leave but they wouldn't budge on the price. So I'm
looking at doing the following upgrade. My only worry is that they
would pick it up as a back door upgrade and prevent me porting my
number from the PAYG Sim to the new phone.

1. Get a Siemens S55 on an Orange Contract ( Free Handset , £25/month
)
2. Port my old Orange Contract Number over to a Voda PAYG Sim (£10?)
3. Port the PAYG number over to the new S55 contract phone.
4. Switch the Orange HSData Service after 4 months (£10/month?)

Would this work???