crm2011 Upgrade a managed solution zip failed

When I upgrade an existing managed solution on crm 2011,

1. The two options 'Maintain Customization' and 'Overwrite Customization' did not show up on the Deployment Wizard.
2. the 'Next' button is disabled.

See the following picture.

Thanks in advance

the following information are from the Import Log. I am confused whether it succeeded or failed?

There are two tabs.

1. On the solution Tab

Status Processed Message Customizations have been imported successfully. Progress [%] 100.00 Duration [s] 43.1

2. On the Component Tab--- one failure

18:18:13.03 SDK Message Processing Step {a3459424-885d-e411-8e4b-005056920049} Invoice.Plugins.PDFInvoice: Create pdfinvoice Invoice.Plugins.PDFInvoice: Create of pdfinvoice Processed 0x80045043 The original sdkmessageprocessingstep has been disabled and replaced.

Finally, I checked changes on the destination environment. I found that changes in managed solution zip have been applied to crm 2011. I guess the import is sucessful, but why?

The import was successful (see the 100% progress). If there would be any error, you will never reach 100%.

The entry in the component tab is not an error, but a warning. During the import there were also changes applied to an existing plugin. The update process will replace the existing step(s) and will recreated them according to the content of the imported solution. This entry tells you simply that it is not exactly the same steps as before.

So what you see here is the normal behavior when importing a solution which contains a plugin.