The correct reading そきゅう uses the standard on'yomi for both characters. 遡 (そ) takes its Go-on reading, common in formal compounds, unlike its kun'yomi さかのぼる. Wrong readings like さっきゅう or さかのぼり mistakenly apply a different sound shift or the kun'yomi. It means to apply rules or effects to a past event, making something retroactive.

遡 (そ) も 及 (きゅう) も音読み。遡る（さかのぼる）の訓読みに惑わされがちですが、熟語では音読みが原則です。過去にさかのぼって物事を適用すること。