1. a King is cosidered the protector and a representative of Vishnu. So our great poets might have described kings having shanku chakram regai .
A poet might have wanted the best reward from RRc. hence he had gone to the extent of describing RRC with Changu chakra regai ( no idea whether an evidence of the same exists.. but stilll proposing)
2. Thadaadhagai Piratti---
I think this holds the key for the changu chakra regai and other things.
Pandya kings take pride that the goddess herself was born in their Clan .
The cholas being under the constant fear and obsession with Pandyas might have felt humbled by the pride of the Pandyas.
So RRC, being one of the dominant kings wanted to assert superiority.
Hence he wanted that he be called an incarnation or an amsam of vishnu and hence changu chakra regai.
Now, the question might arise.. that if he wanted to call himself god and assert superiority over Pandya's he wud have chosen shiva for he being an ardent devotee of shiva.
" purusheshu vishnu " by "bhAaravi" i think might hold the reason for this.
Hence , this changu chakra regai theory might have been fabricated out of Chola's jealousy over Pandya's pride .
I think i can say "ADRA SAKKAI..., ADRA SAKKAI" for my theory.