In Puerto Rico, there are as of Present, 8 Senate Districts - San Juan, Bayamón, Arecibo, Mayagüez-Aguadilla, Ponce, Guayama, Humacao and Carolina - And there has been since the 1952 Redistricting. However, these were not all the districts that existed back then, as Back then Aguadilla had it's own district and Carolina did not.
The Reason for the change is rather simple, As seen in the proposals to maintain the District of Aguadilla, there was not real way to maintain, equapopulous districts in a compact manner in 1972 - whilst maintaining the Aguadilla district. with each proposal taking questionable decitions to get close to the population requirements.
The main proposal, due to Ponce and Arecibo being legaly prevented from being excluded from Aguadilla to maintain those districts (See: la ley de minimos cambios), ended up having to go through the Corrillo central to have Aguadilla meet the population minimum, occuring alongside other controvercial changes (Such as Cataño with Arecibo).
The non-compact and controvercial changes led to an impass on the commission, leading them to eventually scrapping the District of Aguadilla alltogether (at the behest of the president of the commission) and creating the District of Carolina in the San Juan Metro so as to meet the population requirements and the compactness criterion.
The move was controvercial at the time in Aguadilla, with protests seeking to preserve the unpreservable district occuring, resulting in then Senate President Rafael Hernández Colón and the committee seeking to assure the people through PSAs that nothing could have been done(Source).
Since 1972 (as mentioned at the start) all the districts since have remained functionally the same, only changing as little as possible to meet the population requirements. As a result, this makes the 1972 Redistricting cycle, the most controvercial, and one of the most impactful ones in Puerto Rico's Electoral History.
Documental sobre la Eliminacion del distrito de Aguadilla: https://www.youtube.com/watch?v=mijf556EB00
Source 2: http://209.68.12.238/servicioCEE/redistribucion/pdf/LAREDISTRIBUCIONELECTORAmarzo2005-Cambios.pdf
Source 3: https://revistajuridica.uprrp.edu/wp-content/uploads/2016/12/2.RedistribucionElectoral.Denton.pdf
Proposal 1 (PPD 1): https://davesredistricting.org/join/1e9c0b30-afe7-460f-b7ba-3e07a12806d1
Proposal 2 (PPD 2): https://davesredistricting.org/join/6e90f519-9db4-4678-b948-9c88e8f73e9b
Proposal 3 (Villamartinez 1): https://davesredistricting.org/join/e5ef5aa2-f346-44f0-bcf4-8b2b0f3ce12b
Proposal 4 (Villamartinez 2): https://davesredistricting.org/join/6ed7d9df-1f71-434d-8422-3c9244374986
Proposal 5 (PPD 3): https://davesredistricting.org/join/a543f391-473b-4b50-aace-b233d2a4090d
Proposal 6 (PPD 4): https://davesredistricting.org/join/44f407a9-1fa4-4ffe-97de-087affe17183