Is it possible replace CPU from i3 to i5 or i7?

It follows the same procedure as both are LGA 1156 (i3, 15 and half of i7 is, the other half of the i7 family is LGA 1366)

after 1 year i3 to i7 870 working very nice

Did you try to change it? Did it work?

Did you just do a straight up swap or was there anything extra involved(i.e. firmware or graphics)?

"I will be updating my iMac mc510ll/a, intel i3-550, ati hd5670 to i7-870, will let all of you know, ok! would appreciate any input from Jooyoung Park or anybody

Update

So finally yesterday, I got my i7-870 processor and I was able to just swap the i3-550 in my iMac 27'' mid 2010, 11,3 to i7 was just like Renato said plug and play, thank you guys, I have successful upgrade on my iMac and I'm very please to report that the performance on my iMac has been unleashed, I'm talking about mostly about my video decoding tasks, this processor is a beast. Heres is the system in detailed after upgrade, also I used arctic silver as the heat transfer compound as advised.

Hardware Overview:

Model Name: iMac

Model Identifier: iMac11,3

Processor Name: Intel Core i7

Processor Speed: 2.93 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 8 MB

Memory: 12 GB

Processor Interconnect Speed: 4.8 GT/s

Boot ROM Version: IM112.0057.B00

SMC Version (system): 1.59f2"

"You won't be able to go to Sandy Bridge. The i3 550/i5 650 /i7 860/870 are all socket 1156, Sandy Bridge chips (i5 2310/2400/21500, i7 2600) are all socket 1155 - that'd be a different logic board.

In theory, you could start with a new 2011 version with the i5 quad core and upgrade it to the i7 sandy bridge, but that's quite the expensive upgrade to do on a machine if you don't have a reason to.

The 2010 iMac's are 1156 - Pre-SandyBridge only. If someone proves that wrong, please let me know!"