Complex sentence: Poland was initially replaced by Jay Reynolds of Malice, but as the band began working on their next record, Reynolds was replaced by his guitar teacher, Jeff Young, by which time Megadeth were six weeks into the recording process of their third album.
Split into simpler sentences: Poland was initially replaced by Jay Reynolds of Malice, but as the band began working on their next record, Reynolds was replaced by his guitar teacher Jeff Young. By the time Young joined the band, Megadeth were six weeks into the recording process of their third album.