I don't think it's a good idea, i mean China has controlled whole XiSha/Paracel islands it's a un-disputed area, not like NanSha/Spratly island the situation much complex than Paracel islands.
NanSha/ Spratly islands were separated by many nations, including Vietnam and China. This swap islands idea will make XiSha/ Paracel islands as complex as NanSha/Paracel islands, a un-disputed area become a disputed islands, it's not good for China.
And XiSha/ Paracel islands is much closer to China HaiNan province than NanSha/ Paracel islands, it's easy for Chinese to govern and defense islands.
Chinese need oil underwater, do u really think Chinese wanna waste our money and ppl to control those deserted & no fresh water islands ???