To add to this, it is my opinion that it is the Japanese being very shrewd about this.
To get around their constitution, they produce the platform - frigate/destroyer sized military grade vessels; and when/if conflict arises (or if they change their constitution), they can add the relevant weapon and sensor systems to add significant offensive capabilities and make it a very powerful multi-role vessel.
They would not change these vessels into destroyers or frigates. They were not designed for it, and the Japanese have no need for that...they do however, need them to be counter mine warfare vessels.
It is very expensive to add high end offensive weapons and the sensors to control them onto a vessel that does not have them.
The Japanese already have the second most powerful and modern frigate and destroyer navy on earth behind the US Navy. Their Kongo and Atago AEGIS vessels are very powerful and the Janaese indicate they will add two more Atagos. The MK-41 VLS they have on those vessels is designed to fire offensive land-attack cruise missiles if necessary, and it would be very easy for the JMSDF to quickly make these vessels capable of doing so.
The new Izumo Class carriers which have been assigned and are going to be outfitted for ASW work, could relatively easily be modified to embark up to 12 F-35B aircraft, if Japan ever decided to do so.
But the mine warfare vessels are for countering mine warfare. The JMSDF needs them for that. That is "real" combat. The Japanese have not designed these vessels to be some kind of destroyer or frigate in hiding.
The JMSDF already has plenty of front line FFGs and DDGs.
6 x Abakuma Class FFG, 2,500 tons
5 x Hatsuyuki Class FFG, 4,000 tons
8 x Asagiri Class FFG, 4,900 tons
9 x Murasami Class DDGs, 6,100 tons
5 x Takanami Class DDGs, 6,400 tons
4 x Akisuki Class DDG, 6,800 tons
4 x Kongo Class AEGIS DDG, 9,500 tons
4 x Atago Class AEGIS DDG, 10,000 tons (2 now, 2 more planned)
2 x Hyuga Class DDH, 20,000 tons
2 x Izumo Class DDH, 30,000 tons (1 now, 1 more planned)
That's 19 very capable, modern Frigates, and 30 very capable, modern Destroyers. More than any other navy in the world outside of the US Navy. The Japanese spend a lot of time and money exercising and training, alone and with the US Navy. They are recognized as being very professional and good at what they do. As BD Popeye would say, they are "squared away."