"Modernized" means the bodyshell is identical (or very close) to the original model's, but placed onto a modern chassis. To make it simple, these are modern models with vintage bodyshells since the original mechanical aspects were abandoned.
We now get into very subjective aspects since they mostly rely on personal point of view: vintage purists may be upset by these modernized re-releases. On the other hand, fans of the brand or nostalgic people will be pleased to get these vintage-looking models without having to deal with ancient mechanics problems (often fragile and/or outdated).
Honestly, without sacrificing objectivity, we can consider these re-releases as modern transpositions of the original model's philosophy. The modernized versions are sorted in orignal model's order of appearance:
58001 Porsche 934 Turbo RSR (1976)
49400 Porsche 934 Turbo RSR (2006)
49400 modified TA-05 chassis
84431 Porsche 934 Jagermeister (2016)
84431 TA-02SW chassis
Porsche 934 Turbo RSR (2006)
Porsche 934 Turbo RSR Jagermeister (2007)
Porsche 934 Turbo RSR Black Edition (2009)
30 years after its inital release, Tamiya celebrates their first RC model with a modernized version using the TA-05 chassis (4 wheel drive!) in a modified 230mm wheelbase version. The 1/10th scale lexan bodyshell reproduces the original model's livery.
To celebrate the 40 years of the iconic Porsche 934 RSR, Tamiya offers another modernized version, this time using the TA-02SW chassis equipped with loads of options. The 1/10th scale lexan bodyshell reproduces the Jagermeister livery.
There are other modernized versions that were released in the TamTech Gear series: again, the bodyshell is in lexan, but this time at the original 1/12th scale. The Jagermeister livery is a reminder of the static model that was converted into RC to become the very first RC model in Tamiya's history: this livery was also available as an alternative bodyshell spare part back in the days. Last, the Black Edition version celebrates the special limited version Tamiya released in the late 70's to commemorate the first 100.000 sale of their first RC kit.
58002 Martini Porsche 935 Turbo (1977)
The bodyshell is strictly identical to the original's, but in lexan. The chassis is a TamTech Gear.
49154 Tyrrell P34 Six Wheeler (2000)
The chassis is a modified F-103RS. To my knowledge, this is the model Tamiya chose to start their "modernized" re-released program (a few weeks after the XR311 Combat Support Vehicle "original" re-release).
58005 Lamborghini Countach LP500S (1978)
57105 Lamborghini Countach LP500S (2007)
58413 Lamborghini Countach LP500S (2008)
On the first modernized version, the bodyshell is strictly identical to the original's, but in lexan. The chassis is a TamTech Gear. On the second however, which is not officially presented as a re-release, the bodyshell is at 1/10th scale to fit the TT-01E chassis.
58009 Toyota Celica LB Turbo Gr.5 (1978)
58513 Toyota Celica LB Turbo Gr.5 (2012)
The modernized version, which is not officially presented as a re-release, is based on the TT-01E chassis and the bodyshell is now in lexan at 1/10th scale instead of original's 1/12th scale ABS.
58011 Ferrari 312T3 (1979)
49191 Ferrari 312T3 (2002)
The modernized version is based on the F-103RS chassis. Two bodyshells are included in the box, one in ABS and the other in lexan (both with a minor modification to mount the antenna tube).
58020 JPS Lotus 79 (1980)
84122 Lotus Type 79 (2010)
The modernized version uses the F-104W chassis and rubber tires instead of the original foams.
58022 Datsun 280ZX (1980)
92213 Datsun 280ZX Sports Version (2009)
84329 Datsun 280ZX Street Custom (2012)
The modernized versions use the FWD M-05 chassis (RWD on the Racing Master original). Even though the modernized models are claimed to be 1/10th scale (1/12th for the original), the lexan bodyshell is identical, apart from the livery.
58023 Holiday Buggy (1980)
58470 Holiday Buggy (2010)
84212 Holiday Buggy Red Edition (2011)
Modernized versions use the DT-02 chassis, this time propelled by a Mabuchi 380 like the original. Holes were made in the front bonnet to leave room for shock towers.
58024 Sand Rover (1981)
58500 Sand Rover (2011)
58522 Sand Rover Street (2012)
58615 Buggy Kumamon Version (2015)
The modernized uses the DT-02 chassis, but propelled by a Mabuchi 380 like the original. Holes were made in the front bonnet and the rear body part to leave room for shock towers. The "street" and "Kumamon" versions didn't exist back in the 80's.