There are no use to upgrade your CID49 to CID52 because there aren't firmwares for W810i CID52 (only CID49 & 50)
If you use this MAIN than choose FS R4DB005 RED49 from this:
EUROPE_1 English - Portuguese - Spanish - French
EUROPE_2 English - French - German - Sesotho (South Africa) - Zulu - Turkish
EUROPE_3 English - Danish - Dutch - French - German
EUROPE_4 English - Finnish - Danish - Icelandic - Swedish - Norwegian
EUROPE_5 English - French - German - Italian
depending on yours language and in that direction you choose CUSTPACK
Use this 2 tutorials - 1 for unbranding
2 for unlocking (again)
because after flashing your phone will be locked again
Flash your phone using SETool 2 Lite - Top Sony Forums
Remove SIM-lock patch generator - Top Sony Forums