Diaz, 26, hit .296 with two doubles, a home run and seven RBI in 22 games with the Express. The Dominican Republic native has appeared in part of four Major League seasons with the New York Mets and the Texas Rangers. Diaz hit .240 with nine home runs in 37 games with the Rangers in 2007.
Paz, 30, spent the previous two seasons with independent Alexandria where he batted .305 and .323, driving in 124 runs and scoring 138 over 178 games. Through 20 games with the double-A Corpus Christi Hooks, Paz was batting .386 and had nine multihit games.
Cassel, 27, was optioned to Round Rock on Wednesday to make room on Houston's roster for infielder Ty Wigginton. He reported to the Express early Friday.
Cassel, who made his Major League debut and appeared in six games for the San Diego Padres in 2007, was 1-0 with a 5.59 ERA over his two outings with Houston. He picked up a 5-3 win against the Cincinnati Reds on April 24 and allowed three runs over 4 2/3 innings against the Arizona Diamondbacks on Tuesday. He is 1-1 with a 2.50 ERA over three starts for the Express this season.