diff --git a/docs/src/api.md b/docs/src/api.md
index bea4421..78e0ed3 100644
--- a/docs/src/api.md
+++ b/docs/src/api.md
@@ -30,6 +30,7 @@ Optimisers.AccumGrad
 Optimisers.ClipGrad
 Optimisers.ClipNorm
 Optimisers.MixedPrecision
+Optimisers.add_mixed_precision
 Optimisers.OptimiserChain
 Optimisers.SignDecay
 Optimisers.WeightDecay
diff --git a/src/Optimisers.jl b/src/Optimisers.jl
index 07e1ac9..80b4b7c 100644
--- a/src/Optimisers.jl
+++ b/src/Optimisers.jl
@@ -24,7 +24,7 @@ include("rules.jl")
 export Descent, Adam, Momentum, Nesterov, Rprop, RMSProp,
        AdaGrad, AdaMax, AdaDelta, AMSGrad, NAdam, AdamW, RAdam, OAdam, AdaBelief,
        WeightDecay, SignDecay, ClipGrad, ClipNorm, OptimiserChain, Lion,
-       AccumGrad, MixedPrecision
+       AccumGrad, MixedPrecision, add_mixed_precision
 
 VERSION >= v"1.11.0-DEV.469" && eval(Meta.parse("public apply!, init, setup, update, update!"))
 
diff --git a/src/rules.jl b/src/rules.jl
index 224b27a..c1e8304 100644
--- a/src/rules.jl
+++ b/src/rules.jl
@@ -925,3 +925,46 @@ end
 
 adjust(o::MixedPrecision{T}, eta::Real) where T = MixedPrecision(T, adjust(o.rule, eta))
 adjust(o::MixedPrecision{T}; kw...) where T = MixedPrecision(T, adjust(o.rule; kw...))
+
+
+"""
+    add_mixed_precision([T], tree, model) -> new_tree
+
+Add mixed precision to an existing optimisers state `tree` for `model`. 
+If `T` is not provided, `Float32` is used.
+
+Each leaf of the new returned tree will contain a `MixedPrecision` rule wrapping the original rule, 
+and the states will be preserved and converted to type `T`.
+"""
+add_mixed_precision(tree, model) = add_mixed_precision(Float32, tree, model)
+
+function add_mixed_precision(T, tree, model)
+  cache = IdDict()
+  tree = _add_mixed_precision(T, tree, model; cache)
+  isempty(cache) && @warn "setup found no trainable parameters in this model"
+  return tree
+end
+
+function _add_mixed_precision(T, tree, x; cache)
+  ch, re = functor(tree)
+  return mapvalue((ti, xi) -> _add_mixed_precision(T, ti, xi; cache), ch, _trainable(x))
+end
+
+function _add_mixed_precision(T, tree::Optimisers.Leaf, x; cache)
+  haskey(cache, tree) && return cache[tree]
+  fT(z) = z isa AbstractFloat || isnumeric(z) ? T.(z) : z
+  if !(tree.rule isa MixedPrecision{T})
+    if tree.rule isa MixedPrecision # different type
+      rulenew = MixedPrecision(T, tree.rule.rule)
+      statenew = fmap(fT, tree.state)
+    else
+      rulenew = MixedPrecision(T, tree.rule)
+      statenew = (T.(x), fmap(fT, tree.state))
+    end
+    treenew = Leaf(rulenew, statenew, tree.frozen)
+  else
+    treenew = tree
+  end
+  cache[tree] = treenew
+  return treenew
+end
diff --git a/test/rules.jl b/test/rules.jl
index 0455792..800c9e8 100644
--- a/test/rules.jl
+++ b/test/rules.jl
@@ -286,3 +286,31 @@ end
 
   @test_throws ArgumentError OptimiserChain(MixedPrecision(Adam()))
 end
+
+@testset "add_mixed_precision" begin
+  d = rand(Float16, 2,2)
+  d2 = rand(Float16, 2)
+  model = Foo(Foo(d, d2), d)
+  opt_state = Optimisers.setup(AdamW(), model)
+  @test opt_state.x.x === opt_state.y
+  @test opt_state.x.y.state[1] isa Vector{Float16}
+  @test opt_state.x.y.state[2] isa Vector{Float16}
+  @test opt_state.x.y.state[3] isa Tuple{Float16, Float16}
+
+  opt_state_new = add_mixed_precision(opt_state, model)
+
+  @test opt_state_new.x.x.rule isa MixedPrecision{Float32}
+  @test opt_state_new.x.x === opt_state_new.y
+  @test opt_state_new.x.x.state[1] isa Matrix{Float32}
+  @test opt_state_new.x.x.state[1] ≈ model.x.x
+  @test opt_state_new.x.y.state[2][1] isa Vector{Float32}
+  @test opt_state_new.x.y.state[2][2] isa Vector{Float32}
+  @test opt_state_new.x.y.state[2][3] isa Tuple{Float32, Float32}
+
+  opt_state_new2 = add_mixed_precision(Float64, opt_state_new, model)
+
+  @test opt_state_new2.x.x.rule isa MixedPrecision{Float64} # MixedPrecision{Float32} replaced
+  @test opt_state_new2.x.x.rule.rule isa AdamW # no nesting of MixedPrecision
+  @test opt_state_new2.x.x.state[1] isa Matrix{Float64}
+  @test opt_state_new2.x.x.state[2][1] isa Matrix{Float64}
+end