Different plot entirely but both play on the fact that the leads have a link. In Unexpected they share senses whilst in Link: Eat, Love, Kill they share emotions

Both involve the lead meeting a younger version of themself
Both involve some kind of time travel
(Something something 100 letters)

Older male lead becomes young
Tries to develop a relationship with his son (and his daughter in 18 Again)
The difference is that the romance is between his wife in 18 Again whilst Mr Back is with an employee at his company

Both involve the main character who travels to an alternate world where the main character's situation is different in each world

Both detective series.
Both male leads are good at their job
The female rookie helping the male lead (also the same actress playing her)

Though the plot is different, they both involve the older man turning young (with the same face they had when they were younger), developing a relationship with the younger female lead (brought together by fate and met before the ML turned young) and the male lead bonding with their son (with the male lead pretending to be their brother).

Similar body swap concept between older gangster/yakuzaa and younger man
Younger man not good at fighting unlike the older man

Similar concept, both focusing on relationships between in laws especially the daughter-in-law & mother-in-law relationship. The atmosphere is also similar- the male lead's mother initially not approving of their relationship.

Both deal with the issues of Teen pregnancy while still going to school. Both couples love each other very much, who want to stay together but some other people are against it, they both decide to keep the baby.

The difference is that the couple in 'Jenny & Juno' are equals in society and whereas in 'Heard It Through the Grapevine' are not