If i change my hosting server ?

i think server changes doesnt affect in serach engine rankings but the changes of domain will affect.

 

Yes, sometime changing host will make difference in your ranks. Don't forget Google counts a no. of factors to rank you. Not only the back links. Your site location (where is the hosted server is) do matter. If you moved to UK from US then definitely your ranks will have fluctuation since you have some relevant and neighborhood with some other sites which were hosting over co-location or some geo location. Same thing can be applied over PR drop too. Since Google prefers your to have links from a similar demography and (just to say...) if you have moved from US to UK then definitely your back links are from some other location and that might not be relevant in that context as they were before. So, it is possible that your ranks and PR may drop.