domenic@, what's the reaction by other browser vendors for this feature? I couldn't find any information in the spec issue discussion. We need this information to create a chromestatus entry and an intent-to-ship.

Web developer: Positive (I know Polymer team wants this)
Safari: ?
Firefox: ?
Edge: ?

Apple is positive; they posted https://github.com/w3c/webcomponents/issues/710.

Mozilla and Edge were supportive of the feature at the F2F but all discussions were so short we don't really have any strong signals in the minutes. See e.g. https://www.w3.org/2018/03/05-webplat-minutes.html and search for "Imperative upgrades".

#3, thank you for the information.
I created a chromestatus entry.  https://www.chromestatus.com/feature/5967337471606784

Next step: Send intent-to-implement-and-ship to blink-dev.

Posted intent-to-implemnet-and-ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/zCQe7UkR07w

Waiting for 3 LGTMs

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d9efd4223a15ca5db6e840d65538bcba93e8cb84

commit d9efd4223a15ca5db6e840d65538bcba93e8cb84
Author: Kent Tamura <tkent@chromium.org>
Date: Fri Apr 13 04:22:25 2018

custom-elements: Implement customElements.upgrade().

Intent-to-implement-and-ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/zCQe7UkR07w

Bug:  819482 
Change-Id: I5645d5711d7fe1323992533e6c73593ba59d1bca
Reviewed-on: https://chromium-review.googlesource.com/1009450
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550513}
[delete] https://crrev.com/5cf8deb5cd4e3e09de3f5fff6a05f2a4b55a6541/third_party/WebKit/LayoutTests/external/wpt/custom-elements/custom-element-registry/upgrade-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/external/wpt/html/dom/interfaces.https-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/external/wpt/interfaces/html.idl
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.cc
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.h
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.idl

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d9efd4223a15ca5db6e840d65538bcba93e8cb84

commit d9efd4223a15ca5db6e840d65538bcba93e8cb84
Author: Kent Tamura <tkent@chromium.org>
Date: Fri Apr 13 04:22:25 2018

custom-elements: Implement customElements.upgrade().

Intent-to-implement-and-ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/zCQe7UkR07w

Bug:  819482 
Change-Id: I5645d5711d7fe1323992533e6c73593ba59d1bca
Reviewed-on: https://chromium-review.googlesource.com/1009450
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550513}
[delete] https://crrev.com/5cf8deb5cd4e3e09de3f5fff6a05f2a4b55a6541/third_party/WebKit/LayoutTests/external/wpt/custom-elements/custom-element-registry/upgrade-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/external/wpt/html/dom/interfaces.https-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/external/wpt/interfaces/html.idl
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/WebKit/LayoutTests/webexposed/global-interface-listing-expected.txt
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.cc
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.h
[modify] https://crrev.com/d9efd4223a15ca5db6e840d65538bcba93e8cb84/third_party/blink/renderer/core/html/custom/custom_element_registry.idl