As Jeff says, the Guppies are all modified fleet submarines of the Balao and Tench classes (although Barb, a Gato, underwent a conversion of sorts and was given to the Italian Navy). After WWII, the examination of the German Type XXI U-boat convinced the US Navy that it needed a high speed, long underwater endurance submarine. The ill-fated Tang class, based on Type XXI features was planned, but what was originally supposed to be an interim solution until the Tangs arrived were Guppy conversions of fleet boats. The appearance of the Soviet Whiskey class, which clearly had some Type XXI design elements added to the urgency. The Greater Underwater Propulsive Power (GUPPy) program was designed to raise maximum underwater speed from 9 kts to 16 kts, and also greatly increase the endurance at those speeds with more and better batteries. Also, a snorkel would make it unnecessary to fully surface to recharge batteries. The Guppy boats ended up serving much longer than originally envisioned, some well into the 1970's.
The boats were streamlined and the conversions removed all external guns and other protruberances. The bow was redesigned from the "bull nose" to a rounded, streamlined version and a "balcony" with multiple sonar hydrophones added under the lower bow (chin sonar). The conning towers were covered with streamlined sails; initially two designs of a two level "step sail". A snorkel was included, although the design had to be modified to a larger snorkel, as US engines were two cycle diesels and needed more air than the 4 cycle German engines. New five bladed propellers were fitted with beefed up shaft struts. Internal compartments were rearranged with the original intent to double the battery cells from 256 to 512, with a newer high capacity battery design. As with all things govemmental, money became short and many of the boats did not get the battery upgrade. This led to several variants (Guppy 1a, Ib, 2 etc.). More than 50 boats were converted to Guppys and served during the 1950's and 1960's, gradually being replaced by nuclear attack boats.
When Thresher was lost, there were delays in the nuclear submarine program to design and implement SubSafe measures. To fill the gap in nuclear boat deliveries, some of the Guppy boats were given a second Guppy conversion (Guppy III). The step sail (which would leave the bridge crew soaked during runs into the rough North Atlantic and North Pacific) was replaced with a steel framed fiberglass sail (High Atlantic sail) with the bridge raised up to the same height as the masts. These boats also had their hulls cut apart and a 15 foot hull plug installed to give them a separate sonar room and more modern sonar gear. These served on into the 1970's until finally replaced when the last Sturgeons came online.
Comparison as a "before" and "after" on USS Becuna's conversion:
Here's an Electric Boat design step sail on USS Becuna:
And here's a great shot of two Guppies-one with the high sail (second from left) and one (far left) with step sail, alongide of an unconverted fleet boat and a "Fleet snorkel" conversion (streamlined sail, snorkel, but no streamilned bow or other goodies):
Finally, here's a full length shot of a Guppy (USS Blenny) with step sail. Note the rounded off bow shape as well.