Tex. Gov't Code § 853.302

An elected officer who becomes a member of the retirement system on the effective date of an ordinance adopted under Section 852.107 is entitled to prior service credit computed as provided by Section 853.105, except that if the employing municipality has granted updated service credits, the percentage to be used in computing a prior service credit under this section is the percentage of the base updated service credit that was most recently used by the municipality in computing updated service credits.