Android目前的版本只能旧版本升级新版本，无法升级旧版本。

        Android OTA升级主要是ota_from_target_files.py这个脚本，文件目录在build/tools/releasetools，OTA升级判断版本是否可以升级，主要基于时间戳的判断，既然基于时间戳判断，只要将脚本中时间的判断注释即可。

def WriteFullOTAPackage(input_zip, output_file):target_info = BuildInfo(OPTIONS.info_dict, OPTIONS.oem_dicts)# We don't know what version it will be installed on top of. We expect the API# just won't change very often. Similarly for fstab, it might have changed in# the target build.target_api_version = target_info["recovery_api_version"]script = edify_generator.EdifyGenerator(target_api_version, target_info)..............# Assertions (e.g. downgrade check, device properties check).ts = target_info.GetBuildProp("ro.build.date.utc")ts_text = target_info.GetBuildProp("ro.build.date")script.AssertOlderBuild(ts, ts_text)

script.AssertOlderBuild(ts, ts_text)方法是判断时间戳，将此方法注释即可，编译OTA升级包验证，发现下载升级时提示版本不支持升级。

        项目使用的Android版本是9.0，OTA升级会先下载升级包，下载升级也会判断时间戳，只能梳理下载的流程，最后发现在system/update_engine/payload_consumer/delta_performer.cc文件中判断时间戳

ErrorCode DeltaPerformer::ValidateManifest() {// Perform assorted checks to sanity check the manifest, make sure it// matches data from other sources, and that it is a supported version.bool has_old_fields =(manifest_.has_old_kernel_info() || manifest_.has_old_rootfs_info());for (const PartitionUpdate& partition : manifest_.partitions()) {has_old_fields = has_old_fields || partition.has_old_partition_info();}............if (manifest_.max_timestamp() < hardware_->GetBuildTimestamp()) {LOG(ERROR) << "The current OS build timestamp ("<< hardware_->GetBuildTimestamp()<< ") is newer than the maximum timestamp in the manifest ("<< manifest_.max_timestamp() << ")";return ErrorCode::kPayloadTimestampError;}// TODO(garnold) we should be adding more and more manifest checks, such as// partition boundaries etc (see chromium-os:37661)..return ErrorCode::kSuccess;
}

其中if判断则是时间戳判断，将此判断注释即可