Use -- to separate download name and version.

Library/Homebrew/compat.rb

@@ -1,6 +1,7 @@
 require "compat/os/mac"
 require "compat/dependable"
 require "compat/dependency_collector"
+require "compat/download_strategy"
 require "compat/fileutils"
 require "compat/formula_support"
 require "compat/hbc"

Library/Homebrew/compat/download_strategy.rb

@@ -0,0 +1,16 @@
+class AbstractFileDownloadStrategy
+  # TODO: This can be removed after a month because downloads
+  #       will be outdated anyways at that point.
+  module Compat
+    def initialize(url, name, version, **meta)
+      super
+
+      old_cached_location = @cache/"#{name}-#{version}#{ext}"
+
+      return unless old_cached_location.exist?
+      FileUtils.mv old_cached_location, cached_location, force: true
+    end
+  end
+
+  prepend Compat
+end

Library/Homebrew/download_strategy.rb

@@ -181,7 +181,7 @@ class AbstractFileDownloadStrategy < AbstractDownloadStrategy
 
   def initialize(url, name, version, **meta)
     super
-    @cached_location = @cache/"#{name}-#{version}#{ext}"
+    @cached_location = @cache/"#{name}--#{version}#{ext}"
     @temporary_path = Pathname.new("#{cached_location}.incomplete")
   end
 

Library/Homebrew/test/cmd/fetch_spec.rb

@@ -2,10 +2,10 @@ describe "brew fetch", :integration_test do
   it "downloads the Formula's URL" do
     setup_test_formula "testball"
 
-    expect(HOMEBREW_CACHE/"testball-0.1.tbz").not_to exist
+    expect(HOMEBREW_CACHE/"testball--0.1.tbz").not_to exist
 
     expect { brew "fetch", "testball" }.to be_a_success
 
-    expect(HOMEBREW_CACHE/"testball-0.1.tbz").to exist
+    expect(HOMEBREW_CACHE/"testball--0.1.tbz").to exist
   end
 end

Library/Homebrew/test/download_strategies_spec.rb

@@ -238,13 +238,13 @@ describe CurlDownloadStrategy do
     subject { described_class.new(url, name, version, **specs).cached_location }
 
     context "when URL ends with file" do
-      it { is_expected.to eq(HOMEBREW_CACHE/"foo-.tar.gz") }
+      it { is_expected.to eq(HOMEBREW_CACHE/"foo--.tar.gz") }
     end
 
     context "when URL file is in middle" do
       let(:url) { "http://example.com/foo.tar.gz/from/this/mirror" }
 
-      it { is_expected.to eq(HOMEBREW_CACHE/"foo-.tar.gz") }
+      it { is_expected.to eq(HOMEBREW_CACHE/"foo--.tar.gz") }
     end
   end
 end