$ opam upgrade
Everything as up-to-date as possible (run with --verbose to show unavailable upgrades).

The following packages are not being upgraded because the new versions conflict with other installed packages:
  - base.v0.12.0
  - bin_prot.v0.12.0
  - core.v0.12.1
  - core_kernel.v0.12.0
  - fieldslib.v0.12.0
  - jane-street-headers.v0.12.0
  - parsexp.v0.12.0
  - ppx_assert.v0.12.0
  - ppx_base.v0.12.0
  - ppx_bench.v0.12.0
  - ppx_bin_prot.v0.12.0
  - ppx_compare.v0.12.0
  - ppx_custom_printf.v0.12.0
  - ppx_enumerate.v0.12.0
  - ppx_expect.v0.12.0
  - ppx_fail.v0.12.0
  - ppx_fields_conv.v0.12.0
  - ppx_hash.v0.12.0
  - ppx_here.v0.12.0
  - ppx_inline_test.v0.12.0
  - ppx_jane.v0.12.0
  - ppx_js_style.v0.12.0
  - ppx_let.v0.12.0
  - ppx_optcomp.v0.12.0
  - ppx_optional.v0.12.0
  - ppx_pipebang.v0.12.0
  - ppx_sexp_conv.v0.12.0
  - ppx_sexp_message.v0.12.0
  - ppx_sexp_value.v0.12.0
  - ppx_typerep_conv.v0.12.0
  - ppx_variants_conv.v0.12.0
  - seq.base
  - sexplib.v0.12.0
  - sexplib0.v0.12.0
  - splittable_random.v0.12.0
  - stdio.v0.12.0
  - typerep.v0.12.0
  - variantslib.v0.12.0
However, you may "opam upgrade" these packages explicitly, which will ask permission to downgrade or uninstall the conflicting packages.
Nothing to do.