why would you want to upgrade ? if you want to upgrade, you have to consider the motherboard as they are on a different chipset board though 

Assuming you are able to sell if off at RM 500, adding RM 500 on top of it will get you RM 1000 in total, this cannot get you anything beyond Haswell i5 yet, Xeon E3 1230 V3/1231 V3 is priced slightly higher than your available budget (though you may find one which meets your budget  ), while Haswell i7 is all > 1.2k unless you get those OEM/bulk units.
Well it is quite hard TBH, only able to fit a 3GB version of GTX1060 with minor over budget.

Intel Core i5 6402P - RM 754
ASRock H110M-ITX - RM 278
Corsair 8GB DDR4 2133MHz Value RAM - RM 218
Zotac GTX1060 3GB GDDR5 Mini - RM 949
WD Blue 1TB HDD - RM 215
Silverstone SX500 500W - RM 412
Silverstone ML08B - RM 340

Total: RM 3166 (ETech price inclusive of GST)

** Not entirely sure of the 3.5" HDD is able to fit in the case or not but according to the specs sheet, it should be able to sit alongside with the chosen GPU (short card).

If your mobo comes with CMOS jumper then use it to reset, or just remove the CMOS battery for few seconds and put it back, search the Internet on how to do this if you don't understand.
i5 -> i7 grants you 4 extra threads (and slightly higher clock speed), but if your workload is low or doesn't benefit from the multithreaded CPU then it is hard to notice any performance difference.

i7 4790 is the highest end Haswell Refresh CPU already (for non-K) so the answer to your question is yes, it is considered as a good upgrade.