Restructure Active Directory Forest Root Domain

We have a forest root domain of legacy_domain.loc, and another domain modern_domain.loc which exists in a structure similar to:

legacy_domain.loc ----- modern_domain.loc 
       |                       |          \
       |                       |           \
[DC1 & DC2 only]        [DC's, Users,       \
                          Servers,           \ 
                      Workstations, etc.]     \
                                        dev.modern_domain.loc
                                                 \
                                                sub.dev.modern_domain.loc

The only resources in the legacy_domain domain, are the domain controllers themselves. It is also the forest root domain.

Is it possible to make modern_domain the forest root, and then collapse/eliminate the legacy_domain domain?

Basically, when AD was first introduced to the company many years ago, the overall architecture was extremely complex. Over the years, as technology has improved and needs have changed, the superfluous domains were all consolidated/collapsed into what remains today. With everything being in modern_domain, we'd like to finally shed legacy_domain.

Any suggestions/thoughts would be appreciated!